There is no guideline for Deadly graduating to TPK
A TPK is usually not a planned scenario, and so no guidelines are available to show how to introduce one -- even for the "good-natured" purpose that you're asking for, which is to find the upper bound limit of where the party can expect Deadly encounters without all dying off.
I've ran two Invisible Stalkers (CR 9) against three level 5 PCs, only one of which had See Invisibility. These three players had excellent stats, but despite there best efforts, all three characters died. The challenge not only was rated Deadly, they also didn't have the ability to bypass the Disadvantage on attacks from these invisible creatures. However, they actually would have survived this encounter if they had a way of seeing invisible things -- they brought both Stalkers to nearly within an inch of their life before they were killed.
I've been wanting to run a Rakshasa as well, but this monster cannot be presented to a non-magical party or to a party that lacks a spellcaster without level 6 spells or higher. It has only a CR 13, so four level 10 PCs can treat it as a Hard encounter; but when it's immune to nonmagical weapons and spells 5th level or lower, a party may not be able to deal even a single point of damage to this creature.
The above two examples show that a party can die if the DM presents them with a monster they are not equipped to deal with, even if the CR is below Deadly (Rakshasa) or way above Deadly (two Invisible Stalkers).
Below I'll show some exceptions to any TPK-inducing guidelines we might be able to provide.
Exceptional Case 1: Low CR monsters that can induce a TPK to a high level party
Some monsters have been granted a low CR, and yet can actually kill a high level party. As such, many DMs introduce them later on in the game, when the party can handle it (a "rule" I totally break). And so, on paper, encounters with these monsters may be rated as Easy, but it will actually be really, really Deadly.
The Shadow (CR 1/2) comes to mind which, on a hit, reduces the target's Strength score by 1d4. The target dies if their Strength hits 0. This means 4 hits from a Shadow will kill a Str 10 character, on average.
There's also the Intellect Devourer (CR 2), which can force a DC 12 Int Save, subtracting 3d6 from a target's Int score. If it brings the stat to 0, the target is stunned until they gain back a point of Int; so short of Greater Restoration, they're vegetables. On an average roll, an Int 10 character loses this save. Then they can do an intelligence contest with the stunned character and, if they win, the target instantly dies.
The Harpy (CR 1) is another. They approach this differently, but Luring Song is a save-or-die ability when used deviously. In enough numbers, they could lead the party into lava, off 200-foot cliffs, underwater to drown, or into a Sphere of Annihilation (this last one is a bit overkill; but so are Shadows and Intellect Devourers).
Exceptional Case 2: Low CR intelligent foes that perform a Coup de grâce
A DM who uses smart enemies will ignore the tank and head straight for the squishies -- particularly the cleric, druid, or bard, as they have access to healing and resurrection spells. They attack the unconscious character to ensure death. Once the party is down a person, the XP budget is adjusted and a Medium encounter may become Hard or Deadly. Another round of attacks, granted they survive, may allow them to kill the Wizard or Sorcerer next.
While this seems like a dick move, if the DM introduced enough support and foreshadowing for this particular foe to act in this way, the DM could kill the party in this way.
Although, this particular case, I haven't really tried before as a DM. However, I've been on the end of many combats where my character fell unconscious, and had I died then and there for a lack of healing, a TPK would have been introduced. Fortunately my party brought me back. Had the DM focus fired on me, though, there wouldn't have been enough time to heal me. In that instance, when the party and DM was already speculating this was going to be a TPK, there was most likely no other direction we were heading in than a real TPK.
Exceptional Case 3: Using RP to bypass combat with High CR foes altogether
Even the deadliest opponent, as long as they can be reasoned with, can be dealt with via rational discussion. Or trickery. Your mileage may vary.
I've prevented a whole civil war in a game of mine with a single well-placed illusion spell. I did have to roleplay the scene to bring it to a convincing resolution. But the DM was foreshadowing the war for a few weeks, and I apologized after I'd done it. But I think it shows that you can avert almost any crisis with roleplaying.
Best Answer
Carcer and I can independently verify that all your math checks out.
That said, take a deep breath. I doubt this question was prompted by your lack of faith in your own math and reading skills, but instead by your DM's insistence that this encounter wasn't deadly, and this much bold formatting and all-caps in a question (prior to style edits at least) makes it pretty clear that personal feelings are running hot (and probably being hurt) over this issue. If the game is causing you that much stress, maybe it's time for a break, or for a different activity - or for just walking away. No game is preferable to a game prompting this kind of emotional response.
So why are they saying it's not a deadly encounter? I'm not your DM, so I don't have direct access to their thought processes and reasoning, but if I had to guess?
Maybe they don't realize they need to add the additional difficulty multiplier for groups of monsters, as this is something that causes some confusion in 5E. Under this math, the encounter would incorrectly show as medium.
Maybe, and this is what I believe is most likely, you're not talking about the same thing when you say "deadly encounter". They could be using "deadly" to refer to the encounter in plain English - like "trivial" or "effortless" or "frustrating" - while you use it to describe this very specific 5E concept of encounter difficulty. Few things are more frustrating than being separated by a common language like this.
Maybe the oozes were at some disadvantage: surprised, distant enough for your ranged folk to get a licking in before they closed the distance, or something else specific to the encounter that only the DM knew about. Per the DMG, a disadvantage like this drops the encounter one step from deadly to hard.
Maybe the encounter was the first, last, or only encounter in the adventuring day. All of these affect the perceived difficulty of the encounter, despite having identical XP values and theoretically taking up the same fraction of the party’s resources (hp, hit dice, spell slots, etc) per adventuring day. Earlier encounters give you more flexibility in matching the right resource to the encounter (such as using your spell slots to get around damage resistances that would prolong hp-draining melee combat) and more assurance that if things do go pear-shaped you can still handle it, and so they feel easier even if they take more out of the party. The later an encounter gets, though, the less these apply. If you lose almost all your resources to an encounter, it still feels like you made it by the skin of your teeth, even if it was only a small fraction of your adventuring day’s resources. Spending half your Hit Dice and all your spell slots is still going to feel less tense than spending a single hit die, if its your very last one.
If you haven't already done so, I would recommend just talking to them to see if it's any of these. And when you do, give them the chance to explain; they may ultimately be in the right here.