There are two ways to interpret that sentence, and it hinges on a weirdness of how English uses the indefinite article.

Any character or monster that doesn’t notice a threat is surprised at the start of the counter.

The usual interpretation of "a threat" here is that it means "one threat". If that is the correct reading, your question is the result. Is it correct though? This meaning would require that surprise is a relationship between two individuals, so that the dire wolf could be surprised by the rogue but also not surprised by the enchanter.

Is this how surprise works? It turns out, no:

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. (PBRv0.2, p. 69)

Surprise is not a relationship between two entities, it is a state of a single entity. It's impossible to be surprised by one opponent but not surprised by another.

Is there another way to read that sentence about "a threat" that makes surprise sensible as a state? As it turns out, yes.

"English is funny that way"

Another use for the indefinite article, which looks identical to the "one threat" meaning, makes the sentence in question

Any character or monster that doesn’t notice a threat is surprised at the start of the counter.

mean any threat at all, not just one. If this meaning of "a threat" is how it's being used, then that means only creatures who notice no threat are surprised.

Because this reading is perfectly normal English, but ambiguous, we need to confirm the reason by looking for clues in the surrounding text. That confirmation is in the definition of surprise we looked at above: being surprised means being completely surprised, which only makes sense if it happens when no threat is noticed.

So the dire wolf is not surprised, because it did notice a threat, as opposed to not noticing a threat. (See how that makes sense put that way?)

On the plus side, the champion and the rogue don't need surprise to have advantage on the dire wolf, because that doesn't rely on surprise, but rather on being unseen, and being unseen is a directional relationship, not a state.

To answer your question directly, yes. You are correct.

From what I understand though, you are confused about a) the "DC" of your hide check, and b) what happens after you are spotted, and targeted.

Well firstly, a creature's Passive Perception is calculated by adding 10 to their Wisdom Modifier. This means that a creature would need a Wis Score of 16 to spot you with passive perception. This is unlikely. However, the GM may have decided to give them advantage, since they got the drop on you. This means that they get an additional +5 to their Passive Perception score, which means they only requires a Wisdom of 8 in total to still spot you. (10 + 5 - 2 = 13)

Secondly, if they do not spot you, and need to search for you, even if only one succeeds, the gig is up. You've been spotted, and your position has been given away by the monster hacking at you. Even a monster with only 8 Wisdom would still be able to figure out that there's something worth attacking, if one of his friends was having a go at something they couldn't see.