Well not necessarily...
We are told that there is a falsification of reality.
This plays on some Golds like Sojiro and apparently Kogetsuki as well. If the fake armor was there for the sole purpose of telling readers "be careful it's fake, the Golds don't see the armor like that but it's an illusion created by someone and the Golds see the Sagittarius normal" it would be totally absurd because in this case, you might as well put on the real Sagittarius armor and not bother wanting to completely hide your face, etc.... since we assume that it's an illusion then we as readers know it so we might as well put something simpler.
But there, no, we insist on the fake armor with, coincidentally, the full helmet which hides the face, it is frankly not insignificant I imagine and the author wants to insinuate something, hide something and in this case the character inside. That's really how I see it.
For the revelation in the next chapter, I agree with you, the twist will not necessarily be about the revelation of the 3rd boss (well, we may find out) but the twist will be more about the revelation of Sagittarius, that's my personal opinion.