Yeah its the most ridiculous thing. The line of logic they are attempting to build here is that because biological sex can be pretty complex (re; its honestly not actually that complex, but we'll pretend like it is for this argument) then what distinguishes males from females is fundamentally arbitrary, in fact these categories are basically nonsensical social constructs which are completely fungible and up to the varying whims of whatever society, or even individual, interacts with them!
They never quite want to just say it, but the implication is obvious, Male and Female don't actually exist and don't have any reason to be distinguished on any level. Instead everyone is on a sliding scale of 'maleness' to 'femaleness', where for some reason we will doggedly ignore the actual things that matter when it comes to male vs female across basically every species on earth capable of sexual reproduction, that is to say gamete production. But they can't even properly use this reasoning to shore up their own ideology.
If I am presented with someone who displays just about every possible feature that clearly delineates them as a male (that is to say they are on the far end of the maleness spectrum, you know, a good 49.9999999% of the population) then, logically with this in mind, I can just call them a male regardless of their self identity, extraneous hormones and/or surgery, or the fact that they want to make dress go spinny, or at least overwhelmingly more male than female. But I know that this would still be completely unacceptable for this idiots even though I am still adhering to the completely retarded parameters they have attempted to ensure that this argument exists within. Truthfully, intersex conditions have vanishingly little relevance to the trans debate because the vast, vast majority of trans people are not intersex, and the existence of some wiggle room on the fuzzy edges of the boundaries between either sex does not invalidate the overwhelming applicability of the sex distinction to the obviously overwhelming majority of not just people, but every species capable of sexual reproduction bar those that are simultaneous hermaphrodites (and even then, it doesn't really explode the binary since its merely just the ability to produce both gametes co-existing in the same entity).
Let me give a little analogy of why this annoys me, its like running into somebody who really quizzes me on whether or not a donkey and a horse are different species. If I'm defending the position that they are different species there's some things that can be pointed towards that show a certain grey area that raises some questions, like Donkey's and Horses can reproduce and make Mules, and in a very slim number of cases some of those Mules themselves can be fertile, so I can recognize that there exists a little bit of overlap in reality that makes the boundaries a bit more fuzzy than the hardline definition of species (re: the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction) would prefer. After we've talked about this ambiguity between horses and donkeys the other guy then stops me and says 'Aha, now that such an ambiguity has been established that means I am justified in identifying as a giant tube worm and not a human, how is it any different from the difference between a Donkey and a Horse!?'. Just the stupidest most ridiculous jump of reasoning based on extreme fixation on the fuzziness on the extreme edges of definitions like species or male and female that TRAs do all the time, and I'm being a generous with this analogy since the difference between the sexes is far more obvious and clearly defined than the difference between species.