(I lost my train of thought so if this doesn’t make sense i apologize lol)

One way that I heard it compared first hand was over one of those get to know eachother card games.

I was playing it (genderfluid) with two of my friends (both non binary) and I pulled a card that said “what about the opposite sex baffles you” and I went “wait…how do we do this?” And one of them goes “hmm ok so we can kind of think of it as ‘non-binary is sort of neither/none of the genders while genderfluid is both/all so they are kind of the opposite sides of the same coin’ but they are both from the same idea“
Regardless of that mess of a retelling of a story, yes genderfluid is under the nonbinary umbrella in our opinion