To be generous I would say maybe some people have a disorder and that causes them to state they are non binary, but the implication isn’t the other way.
Similarly a mental disorder may make someone obsessed with levels and coding 80hrs a week!
Surely it's only reproductive behavior that is actually sex-specific? For females that would be pregnancy, gestation, birth, and so on. For males, ejaculating semen.
Other behaviors can be performed by either sex, wouldn't you agree? So when someone says they are "non binary" because they don't perform some extreme of femininity or masculinity, based on whatever cultural sex stereotypes they've been exposed to, they are one amongst billions of other individuals who also don't do that, aren't they?
This is as absolutely true as saying elements within the universe are fundementally binary, an atom is either hydrogen or helium (with the exception of atomic aberrations).
The math is the same, 2% of elements within the universe are other than H or He, and 2% of human births have chromosomal aberrations.
To be clearer (and fair to the context): "Pure" intersex is
conditions in which chromosomal sex is inconsistent with phenotypic sex, or in which the phenotype is not classifiable as either male or female", stating the prevalence of intersex is about 0.018%
FWiW I believe there's a bit more to behaviour than pure genetics but it's a hard one to pin down; outside of western society there are many places with a long history for not caring about gender nearly as much as abrahamic westerners seem to; the Bugis, pacific sistergirls, etc.
From a datascience perspective there's a non zero number of actual people in Australia that are born intersex and neither male nor female at birth to warrent a non specific gender entry on passport databases .. as a matter of cold pragmatic fact divorced from any "woke" and|or "culture war" politic posturing.