As a nonbinary trans person, I'm going to have to disagree with pretty much this entire post.
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First off, yes, TERFs do conflate sex and gender. They view gender when expressed as distinct from sex as something to be abolished - but that does not equate to them actually being critical of the idea of gender. TERFs believe in a world where AMAB people are men and AFAB people are women. (And intersex people get to be whatever their doctors / parents want at that particular moment.) That's why TERFS are fine with trans mascs & butch cis women (among others) being kicked out from women's restrooms - it has never been about sex. Any form of gender non-conformity is the enemy. "Sex-based differences" is a smokescreen for what TERFs actually want to do, which is return to an era where there was no difference between gender and sex.
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The nonbinary and trans communities are not two distinct things. They can, and very frequently do, overlap. TERFs are fundamentally not NB friendly because they view all forms of non-conformity to one's assigned gender as indicative of mental illness. They may humor nonbinary folk who are fine with being treated as woman-lite / man-lite, but it is only to weaponize them against the trans & nonbinary people they hate even more.
Being a binary trans person does not mean that you 'only believe in two genders', it means that you personally identify with one of the two binary genders. A binary trans woman is not a TERF, she is a trans person who identifies with womanhood and is comfortable with that role. But ultimately nonbinary people and binary trans people have more in common than they have apart, because we are all subverting the established gender binary, whether it's to traverse from one side to the other or to break out entirely. (And, as I said in my previous post, "nonbinary" and "trans" are not two distinct things - nonbinary people are included under the trans umbrella by default, and plenty of nonbinary people physically transition. I myself started out identifying as a binary trans person and then felt less like I needed to conform to the gender binary as my dysphoria diminished.)
There is some friction between binary trans people and nonbinary (trans) people, but IMO it is no different than some cis gays being biphobic towards cis bisexuals.