I think what people call gender today is a combination of temperament and personality mixed with the person's surrounding culture. Mix those together, and you get the ambiguous "gender," which can then be wherever you place yourself at any time according to those factors. Since they're constantly shifting, pretty much anyone can reasonably identify nonbinary, genderfluid, they/them, etc.

A lot of people subscribe to it, but it's not a framework I find particularly useful because it over-classifies people's behaviors according to traditional gender norms and can cause more confusion than it solves. Personally, I identify as my biological sex, but many parts of my personality and how I deal with things could be viewed as feminine, and many others are very masculine. By anchoring my gender to my sex, I free up my mind to think about my behavior in terms of what really matters, like my morals, beliefs, motivations, goals, etc.

I have no qualms about the gender identity framework per se, but I also think it's being over-utilized and weaponized, which is another reason I don't want to apply it to myself.