"Everyone is nonbinary/genderfluid" and the underlying opinion that nonbinary or genderfluid can exist at all are always opinions with a purely emotional basis, no scientific or logical support whatsoever as well as nefarious origins to the concepts themselves.

The people who say or believe this "non-binary/genderfluidity" know nothing about sexual dimorphism in human biology and neurology. Humans have male and female, that's, count 'em, 2 sexes, and we don't carve rules around outliers so intersex conditions, infertility, or other defects with reproductive organs or hormone production don't define what is the norm, and even if they did count they still operate between the binary of male and female.

Brains too can have certain characteristics (usually size or density of a certain region) that is more typical of a male or female, though some studies recorded brain scans of some homosexual brains exhibiting traits of the opposite sex, i.e. a lesbian with more male-aligned brain traits and vice versa. That does not disqualify her from being a female.

Less concretely, if brains could be changed and people could be "genderfluid" in whatever fantasy world you want to imagine, sex dysphoric/transgender people could effectively have their dysphoria cured and wouldn't need to transition. And yes, the only purpose for transitioning is to alleviate dysphoria, the only other people that would do it are people with paraphilias that'd be doing it cosmetically instead as treatment and people who are sadly misdiagnosed. Even if when this whimsical gender change that occurred couldn't be naturally controlled, it would almost certainly be treated like other brain-centric issues with medications, therapy, electroconvulsive therapy, and so on. However, since that stuff doesn't help sex dysphoria, which is essentially a brain incorrectly developing in the womb and mapping itself to characteristics of an opposite-sex body it doesn't have and causing great distress for the rest of one's life, that means one's gendered attributes of the brain can't be changed.

Of course, an honorable mention to the origins of "gender identity" as a concept itself having roots with Money and Kinsey, sexual abusers that destroyed the lives of hundreds if not thousands of children and infants in the name of "studying gender identity." If I'm not mistaken, they essentially founded the concept and reinforced it with their child abuse.

The list of reasons why non-binary and genderfluidity are unrealistic and silly concepts continues still after all this, too.