You are again asserting that someone has to transition to something rather than decide it’s a better descriptor for themselves than another term.

Someone may simply feel non binary as a high level and honestly descriptor of their gender. Which is to say that they feel the gender implied by their sex is a bad fit.

The existence of some people that take this to extreme labels that seem unlikely to be distinct and noteworthy classifications does not invalidate others who take more moderate steps that are probably genuinely more accurate assessments.