i would say they describe two separate axes. “agender” is how much you identify with gender at all, whereas “gender non-binary” describes your experience of gender, when/if you have it (which is quite a broad term, with the only definition being “not the binary experience of being a a man or woman”)

to elaborate, there are other terms e.g. “demigender” or “genderflux” that describe different points on the “how much do you identify with having gender at all” axis (these are like, partially identify with having gender, but not totally)

so one could be agender or demigirl/boy depending on how much they relate to gender and what they feel their gender is when they do, for example (usually, in this type of situation, someone can still be classified under the bigger umbrella of “agender”, even if they do experience gender sometimes)

(also, the terns might seem like a lot, but a) they’re not super important, and b) they’re mostly learning root words and modifiers, e.g. “a” is “without”, and “gender” is… gender. “demi” is some, “flux” is… flux, etc)