Floisme I'm not sure it is even true.
It's like Schrodinger's transwoman. Simultaneously there and undetectable. Except it doesn't account for the fact most women can clock most transwomen, the fact many women would have been to afraid or uncomfortable to challenge, the fact that trans identifying males in full "drag" were far rarer in the past, and despite there being so few mtf individuals, we've somehow all peed next to one in the past, and this has gone on for decades?
None of it seems very likely.
It's more that we can't prove it hasn't happened, rather than they can prove it has.
I mean, I could say that lizard people have lived alongside humans for hundreds of years, and when you ask me for proof, I say "see? You couldn't even tell they were there. That proves I'm right. And the fairy people too!"