It's been used way before the nsfw stuff and the advent of AI.

Back in the days if you were doing an AMA with a celeb, the picture proof is the celeb telling us this is the account they are using. Doesn't need to be their account and was only useful for people with an identifiable face. If you were doing an AMA because you were some specialist or professional, giving your face and username doesn't do anything, you need to provide paperwork to the mods.

This is a poor way to police fake nudes though, I wouldn't have trusted it even before AI.