The thing is that we all know what Jenner and other trans women are "coming out as", even if we disagree that it is possible for them to actually be that thing.
What does "coming out as non binary" mean?
It's like a negative identity. You're saying you don't identify as a man or a woman. OK cool. So what, you're identifying as nothing? (Of course, there are still two types of nothing, the male type and the female type, and so even being non binary comes in two binary options.)
The whole thing is just deeply silly.