Most times, when people talk along the lines of "woman is hard to define and sex isn't as simple as YOU think", they're referring to a few rare DSDs. They're particularly fond of misusing Swyer syndrome and de la Chapelle syndrome as evidence that any fully grown adult human male should be accepted as a woman and allowed full access to women's spaces and women's sports if he says the magic words.
It's a little interesting that Upton didn't deliver a lecture about the actual biological complexities of sex development in humans, which would be irrelevant to his situation but would at least show that he'd been listening in medical school, and talked a lot of nonsense that's as impossible to pin down as auras and meridians. It's as if he got his medical education in a parallel universe.
I expect more of the same airy-fairy obfuscation in response to efforts to get him to square the circle of providing evidence based medical management, or even basic safe care, for medical conditions that are sex specific or present very differently in men and women if he regards biological sex as nebulous. He will have assertive non-answers for "biological women" like himself with prostatitis and "biological men" with pregnancy complications.
The fact that he's got an answer to everything that's broadly consistent with what he's already said doesn't mean he's honest or even sane. People with fixed delusions (this is an example, I'm not suggesting a diagnosis) can be remarkably skilled at rationalising their bizarre and firmly held beliefs. It does mean that trying to catch him out in an inconsistency is unlikely to be a rewarding examination technique.
I don't know whether he's mad or bad, but whichever it is, he's doing it well.