That's the fundamental level but also, on a practical level, no, there is not a distinction between lying about your sex/gender and simply not providing info that you're not obligated to provide. Cis society does not let you get away with just not providing info. It is basically impossible to successfully remain closeted (or stealth) without sometimes saying something directly untrue about your sex/gender - avoiding the question will itself make people clock you as some degree of gender-nonconforming, and will often get framed as attempted deceit anyways.
First of all, thank you for the explanation. I appreciate the detail and clarity.
Reading your post, it bacame clear to me that I was looking at this discussion as being general, rather than about Mitsuki, specifically, and that I was thinking about it from a queer perspective rather than a heteronormative one.
Frankly, I think most straight people would have their minds blown by the existance of stone butch trans women.
"Whaaaaat?" they'd say, "Isn't that the same as just being a man?"
In regards to Mitsuki, the suggestion that Mitsuki is engaging in any form of deception is perfectly in line with your explanation, so I find myself moved to agreeing with your position.
As for the issue of having a queer/heteronormative perspective, I do still think that a similar, generalised statement coming from a queer person could be up in the air. Honestly, far too many people within the community are shitty to bisexuals, treating them as traitors for being straight passing, so being transphobic is far from out of the question. That said, since the queer community is more open to gender nonconformity, I think that similar statement could be approached with some measure of charity.