iridescent_slime said:
This is like saying "bacon isn't meat, it's pork".
All "trans" means is not identifying as one's birth gender. As nobody is assigned a nonbinary identity at birth, nonbinary is effectively a subset of trans.
Their gender is, by definition, what they identify as. It's their birth sex that doesn't match. And the existence of intersex people proves not everyone is born as part of the binary, though they will often get treated as a boy or girl based on the parents' convenience.
Also, while it may be technically true that being non-binary would fit the definition of being trans for most people, not everyone in the non-binary community classifies themselves as trans, most likely because of the nuances among those communities. An agender person (who might not even consider themselves nonbinary) would have different thoughts, needs, and priorities from someone desiring a sex change, for example. Classifying gender identity is much more complicated than classifying meat.