Or you don't understand him.
He's not very clear, so that's possible. He sounds more angry than coherent on the subject. TBH, I'm not sure he knows what he means or can speak rationally on this. The quote you linked to says:
the notion of “biological sex” as a fixed binary that exists in contemporary political and legal discourse was invented to regulate trans people out of categories.
Maybe he means gender identity rather than sex, but then that seems to be undefinable and based on stereotypes (which I think are harmful). But if he means gender, he should say that.
but they are relevant when you are trying to determine the biological sex of people on an individual level.
Note - it's an individuals that reproduce and ultimately make-up species (& species sometimes actually do have fuzzy boundaries
No, I would write a paper that this species has 2 sexes, but that in some individual critters their sex may be unclear/undefined. In other words: fuzzy.
And the reviewers would not buy that conclusion. You're conflating normal variation with a deleterious mutation. Again, we can now almost always discern which of the two sex pathways has been perturbed. Moreover, It's not a requirement that each individual has a sex. It sounds like you are talking about sexual identity or need a different word for what you're talking about.
There is a binary at core of mammalian reproduction: one oocyte and one sperm pronucleus are required for development. That is a binary, and how I (& many biologists who work in relevant areas) would define the two sexes.
Non-functional variants are irrelevant to that definition.
To many of the individuals involved, it certainly seems random. And it is not merely an observation, often surgery is involved.
... long before they could even consent, it should be noted.
There are obvious parallels in the ways their lives unfold. And there have been many attempts to use intersex individuals to try to study how "gender identity" develops.
I agree that the situations those with DSDs was (occasionally still is) handled poorly (particularly genetic males - "easier to dig a hole than make a pole").
That being said, the observation of newborn sex is not controversial >99% of the time & has not been demonstrated to have anything to do with trans people. E.g. if trans-people had malfunctions in some form of steroid hormone synthesis (as in CAH, the most common spectrum of DSDs- & note that CAH can be fatal) or a major part of sex-determination, we'd know about it.
I understand the temptation, but I don't think it's right to conflate DSDs - which have known mutations and typically result in infertility and often other issues - with people who feel their secondary sex characteristics do not match their brain/gender identity. For that matter, even lumping DSDs together is a stretch.
What would you think if a trans-susceptibility genetic variant was identified in a gene associated with multiple mental illnesses?