I wonder how much of my difficulty understanding nonbinary identities comes from having been in sex work for sooo long, where the genitals you have are incredibly important to how you're treated and the role you play.
In this context, if an afab said she wasn't a woman, id be like, that's nice but that means nothing. People are still going to pay you because of the way you look. Saying you're not a woman has no impact on all the things that are important about being a woman
This would maybe predict that nonbinary identities are more common in more gender-equal worlds, or worlds where there's a strong insistence that genders be treated the same.
It's interesting that you're wondering that because my perception is that non binary people tend to come from a perception of oppression, therefore a stronger enforcement of gender roles, whether actual or perceived.
This is true - I'm thinking of one nonbinary person I know who I believe is nonbinary out of a reaction to intense gender trauma, where their gender roles were used as a weapon against them and now they're nonbinary in an attempt to deal with that pain.