This line seems to have been swallowed whole by so many people, particularly after comments from the BMA after the SC ruling.
I really want somebody to call them out on it.
Rationally, what they are saying is that in a small number of cases a DSD may not be detected. However, of all the protected characteristics, sex is probably the most straightforward to determine.
The impression they give is that sex is all about chromosomes (and is completely detached from sexual reproduction), so no individual can have any way of knowing their sex, without a chromosome test.
It's all such obvious rubbish, but people don't engage their brains.