They completely omit the fact that XX cannot mate with XX nor can XY mate with XY. That is how we define sex.
Before we understood genetics, yes of course we used other obvious visual cues - our bodies. It was still a scientific observation at that, but scientific all the same.
Yes, some people are born with abnormal configurations, but they do not disprove our definition of sex.
The arguments being used here are classic straw man arguments, where some truths are mixed in with massive assumptions (or downright misleading information) in the hope that the myths flow into the mindset as easily as the truths. They are essentially claiming that the XX and XY chromosome classification is an erroneous one, and that sex is / can be defined by other factors too. None of which are scientifically proven I hasten to add.
It's bollocks.