You're right, it's more of a "sex reveal party" technically.
Celebrating that as proof that your child will later identify as a girl only reinforces unfounded societal expectations that their gender identity must correspond with their genitalia (which does kind of imply not supporting them if they identify with a another gender identity later)!
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.
People just want stupid milestones to celebrate their babies coming into the world.