You're right, it's more of a "sex reveal party" technically.
But nobody calls it that, and that's the problem.
I don't think it has anything to do with how the child will later identify, more about their present sex status. I have no doubt that if it was technically possible, people would do "eye/hair colour reveal parties". It's a redhead! And if they grow up and wear lenses/dye their hair purple - that's all good too.
By calling it a gender reveal party, you're making it about gender. Not to mention the colors, the "it's a boy!" balloons, the whatever else is there-- it's all about gender, not sex. And yeah, obviously, this doesn't impact the baby. But a parent who so early celebrates their child's assumed gender identity, purely on the basis of the child's sex, is a parent who will be less receptive to a child that later identifies in other, "unexpected" ways.