This is such a terrible argument. Genetic abnormalities do not make the rule incorrect.There are plenty of chromosomal anomalies that don't fit that statement, there's also people with intermixed organs to consider.
It is entirely correct to say humans have 2 arms, 2 legs, 2 eyes, 10 fingers. But just because there are humans who don’t have have 2 arms or legs or eyes or 10 fingers doesn’t make them not human and it doesn’t make it incorrect to say “humans have 10 fingers”. There are 2 biological sexes in humans. The exception of intersex people doesn’t make this any less true. And this futile argument to apply to transgender people is weak af especially because they’re trying to argue gender is not associated with biology!