I think that the problem is conflating the hiding of relevant story related aspects and the hiding of the unknown.

Hiding the unknown is totally cool- in those two questions that you link (and one of them is mine), it involved hiding what the player was using, and what the player was rolling. That takes away player agency, and interferes with one of the tenets of Fate.

However, use of descriptions to convey dread and obfuscate the nature of an opponent- that is dealing with the unknown. As long as when that opponent intersects the player's agency they understand the nature of what they're dealing with, you're in clear territory.

To take an example: In one of my games, the bodyguard for the person that the PCs were trying to protect turned out to be working for the Big Bad. He had aspects that would point to his cover that were obvious, i.e. Disdain for his Charge. He also had aspects that were not as obvious that revealed his loyalties, i.e. Divided Loyalties. He also had abilities that were far outside of what a bodyguard should have- this is where the ladder and their descriptors come into play when describing his movements. Instead of saying that he used a Melee +5 to block the players attack, I instead say the total that they have to beat. Then when describing it, depending on how obvious I want to be about his abilities, I might say that "his defense was far above average- one of the most superb examples of wing chun that you have ever seen. Or for less obvious uses, I would just describe it.

The player has the agency to use his fate points as normal to overcome the DC. But they don't know whether I rolled well, the NPC was very skilled, the NPC used Fate points, or a combination of the three. But it does become obvious in the dialog that something is going on, and if they player wants to guess/use a skill/invoke an aspect, they have the information that they need to make it happen.

In this way you're able to maintain the air of mystique around the opposition, and in many cases, ratchet it up more than if you didn't give the information at all. After all, what you're really trying to engage is their imagination and their mind- this information sometimes makes them freak out more than just not knowing; I've seen it in action.

Combine approaches and extras to create different narrative justifications for powered and unpowered actions.

Because of the limitations of approaches this probably won't be really viable for a long-form campaign story lasting months-worth of sessions. Still, if you're going to be playing shorter games (a month per campaign, tops) this is an elegant option. If you want to use skills, read this anyway because I'm going to bring skills into it at the end.

I've experimented with this for a werewolf game, actually: by using approaches, your characters can have the same problem-solving attitudes regardless of their form: a Forceful cowboy turns into a Forceful werewolf, and a Clever schoolgirl becomes a Clever magical warrior. When they change form, the narrative shifts to give them justification for using their approaches in more magical and combat-oriented ways: a cowboy can't bite people but a werewolf can, and a schoolgirl can't create magical illusions but a magical warrior can.

(This doesn't work with skills because skills represent what-things-you-can-do-ness while approaches represent how-you-do-things-ness.)

You can represent this shift narrative by clever use of aspects and extras. Aspects should generally be phrased so they're relevant in both forms, which can be difficult but gets easier with practice. It's nigh impossible to give generic advice for specific aspect creation needs like this; the best I can say is that focusing on personality and relationships makes it easier to keep aspects relevant between forms.

Now, extras! There are many ways to craft extras around this. (Extras are for when you want to give effects that stunts can't handle, either because it's too many stunt's-worth of effects, or because a single effect is too complicated or powerful for a stunt to handle gracefully.)

Extra: Moon Infusion
Permission: An aspect indicating your magical nature.
Cost: At the start of each session I'm in, the GM's pool of NPC Fate points increases by 1 for each action I can use magically.
Benefit: Because I am secretly a champion of the Moon goddess, once per session I can reveal my warrior form. When I do, I gain the aspect Infused with the Light of the Moon (with one free invoke) for the rest of the scene.
This gives me magical context for using my actions (like flying, and shooting rays of cleansing light). When you take this extra, pick the actions (Overcome, Create Advantage, Attack, Defend) I can use magically.

I chose to make the extra's cost a "make the NPCs stronger" effect (increasing NPC Fate points) rather than a "make the PC weaker" effect (reducing PC Refresh) because, frankly, it's more interesting to face stronger opponents than to have your own power balanced out. In play it's effectively similar: the opposition scales according to the power the PCs bring to the scenario. The exact cost may need a little tweaking depending on your game. I've borrowed the basic concept from the atomic-robo RPG, which is a wealth of resources for this sort of thing.

Don't worry about including "running out of power" type mechanics in these extras: that's what consequences are for. Just as a gunfighter might take a mild consequence of All out of ammo, a magical girl might take Overcome by doubt or Cut off from my power.

But what about skills?

I started with approaches instead of skills because they're easier and more obvious to use with transformations.

On the face of things it's still pretty straightforward: Just as having a loaded gun lets you use the Shoot skill, having sparklemagic attacks lets you use it too. However, a schoolgirl probably doesn't have a lot of ranks in Shoot, so we need a new level of complexity in representing the transformation if your game uses skills instead of approaches.

Extra: Moon Infusion
Permission: An aspect indicating your magical nature.
Cost: At the start of each session I'm in, the GM's pool of NPC Fate points increases by 2.
Benefit: Because I am secretly a champion of the Moon goddess, once per session I can reveal my warrior form. When I do, I gain the aspect Infused with the Light of the Moon (with one free invoke) for the rest of the scene. This gives me magical context for using my actions (like flying, and shooting rays of cleansing light).
My abilities are different when I'm a warrior: when you first choose this extra, shuffle my skill ranks into a different configuration representing the talents of my magical form (the new configuration must still follow all the game's rules about skill ranks and caps). Whenever I reveal my warrior form, my skills change to their new configuration. They return to normal when I do.

Now we've got a girl whose abilities radically change but her aspects stay the same--so she's still the same person, but she has a different set of competencies when she's transformed.