well, I'm guess the people saying it means assigned sex don't think they are.
if your genitals determine your sex and your identity and your roles, this is way less complicated.
"X or Y? well, X is easier to understand, probably because it's what I was taught as a kid, so definitely X"
It's "Occam's razor: THE POLITICAL PHILOSOPHY!"
but the problem is that often people say "gender" when they're complaining about that kind of gender role stuff and it's interpreted as "gender-identity" that they're complaining about
and in that perspective, "gender is bad" can be read as almost a personal insult.
because they're interpreting gender in the role-sense: society telling them what to wear, do, feel, based on their gender.
if I'm saying things like "GENDER WAS A MISTAKE" I definitely mean it in the role sense.
But you having whatever gender you say you have? great! Enjoy, and I'll do my best to respect your identity.
So I'm supporting the "gender is personal, not role-based" point of view.
If that makes sense.
Just that equally you shouldn't have to accept them, and you should realize that it's a choice you can make.