Okay, let's run with it:

Hydrogen and helium can be determined scientifically through observation. You can't just point at a gas and claim that it's hydrogen without first determining the makeup of the gas scientifically.

They're claiming that this is an accurate analogy for gender identity.

So then the logical conclusion of this argument would be that a person is non-binary until we have scientifically determined their sex (and therefore their gender). Given how rare it is for someone to undergo sex testing, this would mean that the overwhelming majority of people are of indeterminate gender, especially when considering the entire history of the existence of humanity.

That would mean that Dick Dork's claim about molehills applies to both sides of the traditional gender binary and that it should be disregarded as a statistical outlier.

Obviously none of this actually makes sense, for reasons that I'm not going to bore anyone here with, but it's impressive how these facks & logick types will immediately agree with any statement that superficially affirms their worldview, even if agreeing with it would undermine their entire argument upon closer examination.

(I use this argument in a distilled form to bait the "gender is a scientific fact" types - I start by asking them their gender or assuming their gender based on pfp/name etc. when they tell me their gender I ask them when they had this determined scientifically and if they identified as non-binary until that point.

For people who are at least not an irredeemable shithead about it, I might actually engage in a discussion with them and ask them that if they got results back tomorrow that they proved they were biologically the opposite of their gender identity, would they immediately change their name, the clothes they wear, and the pronouns they use to refer to themselves by. Obviously this is a near-impossibly difficult pill for them to swallow [almost like gender is something that you feel as your identity and it's not your chromosomes or your hormonal makeup...] but it's really interesting to observe the beginnings of an earth-shattering realisation for these people when they begrudgingly admit that they wouldn't be able to just wake up one morning and swap to a completely different gender [and to likely have their sexual orientation completely upended in that process as well].)