The source I cited used the phrase "birth-assigned sex", and I quoted that source accurately.
The phrase is an archaic one that is functionally synonymous with "[biological] sex observed at birth". A perfectly fine and normal thing.
Recently, some dickheads have decided that what society really needs right now is to equivocate the concept of biological sex as much as possible, for bizarre sociopolitical reasons we won't (can't, really) go into here. You know where to find that discussion if you think you have a coherent argument to make on that point.
Anyway, these dickheads have been working very hard to convince us that "sex assigned at birth" means that biological sex is not a scientific reality, that it is ambiguous and not objectively observable, and that when a physician "assigns" sex to a newborn he's just hanging a temporary label on a potential social construct that may or may not apply to the newborn as they mature.
This is, of course, obviously toxic and antisocial nonsense. Of course the mutable social construct exists, but the reality of biological sex at birth (and forever after) is still an objective, observable fact.
Which brings us back to pronouns. When I used a sexed third-person pronoun to refer to someone, I am referring to him based on my observation of his biological sex. His complaint, then, about having a preferred pronoun, amounts to a demand that I pretend to ignore his observable sex, and instead refer to him according to his current headcanon about the social construct he embodies. Personally, I prefer to stick to the much more useful, perfectly cromulent, and time-tested practice of third person pronouns referring to observed sex rather than fiat headcanon. EC, on the other hand, prefers to sidestep the entire mess by using gender-neutral pronouns. I think that's also a valid strategy.
What I don't think is a valid strategy is arthwollipot's insistence that he cannot be properly referred to in gender-neutral terms. That he must be referred to as a "he". Not because he's biologically male, no. That would undermine the entire toxic, antisocial program to equivocate biological sex as much as possible. No, he has to be referred to as "he" because of some undefinable, un-quantifiable, and frankly irrelevant social construct. The only thing that happens when EC refers to arth as "they/them" instead of "he/him" is that somewhere a gender essentialist doesn't get their wings.