My understanding of nonbinary is it, in practice, most commonly means "person for whom it is very important that others do not assume they share the stereotypical traits of their sex"
hmmm idk why this seems wrong, i think opinions about perception is like... usually part of it but not actually necessary? like a lot of enbies are like "oh since we now culturally have the option of going by they/them pronouns, i'd prefer that" but dont otherwise care much
but probably when one sees an enby *in practice* (pimply plane.jpeg?) then it's bc it's very important that others don't assume this
bc it takes a lot of effort to successfully signal being a gender that's not really recognized by most people
fwiw to me personally I do not parse enbies as a third (or neither) gender, in my view they are exactly their original gender and trying to do something different that would probably be better described as not actually having to do with gender
like "unfeminine woman" seems like a more appropriate descriptor than "not a woman" for all the nonbinary afabs I've met.
Also nonbinary ppl much more closely match cis ppl of their birth sex in all my surveys than other enbies of the opposite sex, minus stuff like politics