Should have figured you'd cite a known anti-trans organization that distorts scientific reality to make a point. (oh boy, I sure do wonder why they have an article titled "8-Step Action Plan to Eliminate Gender Ideology", surely because they actually understand broadly accepted and heavily researched science and not because they just want to make a political point while sounding scientific!)
They think sex can only be determined by gametes. This ignores the fundamental reality that many humans do not produce either of them while retaining wholly ambiguous or non-ambiguous reproductive organs, that some humans can produce both (albeit rarely), or that some humans lose or gain the ability to produce one throughout their lives. (not producing ova until puberty? Tough luck, guess you're not a female! Body hasn't fully developed internal reproductive organs until later in life? Guess you're sex-less until then! Does your body never produce sperm or ova because of a genetic issue? Guess it's impossible to assign you a sex!)
It also ignores the fact that we tend to classify sex based on phenotypic characteristics. If someone has a penis, generally masculine face and fat distribution, but XX chromosomes and is still able to sometimes produce ova while not producing sperm, even if they're missing the rest of the necessary reproductive functions, for all intensive purposes, you would call that person male. If every other part of their body is typically male, there is no reason to continually insist that person is actually a female because somewhere inside their body they can produce ova that don't do anything.
If sex is determined by gametes, there are exceptions to the rule that can't be classified solely as one or the other.
If sex is determined by chromosomes, then any exception from XX or XY disproves the rule.
If sex is determined by phenotypic characteristics, then we see a spectrum in how they present.
This also simply ignores the fundamental reality that even if you can oversimplify a complex condition into one of two more common options, it doesn't mean that is correct or accurate to do so.
To use the analogy I used before, you could say that all numbers for simplicity, should be rounded to either 1 or 0, even if it's 0.9, 0.2, or 0.2398547293875. That could be useful shorthand, and it could generally describe semi-closely how those numbers would operate within a broadly binary system, but at the end of the day those numbers are not 0 or 1.
Claiming that "0.9 = 1" would be stupid.
Claiming "0.9 is basically 1 so why bother ever giving it a different label" would be stupid.
Claiming "0.9 is close enough to 1 to not make a huge difference in outward perception and day-to-day use, so we can rely on it for shorthand while understanding 0.9 is not 1" would be very reasonable.
You cannot look at a spectrum, say "they're still within the other 2, therefore there's only 2", and call it a day.
It's true these conditions can present similarly to one of the two typical male or female sets of characteristics, or that they can derive from what are often the typical chromosomes of either group. It's not true that they are male or female and there is only a binary and nothing else.