the programme is alluding to the fact that Caster has XY biology, but has a DSD which means that she can only partially synthesise testosterone.
Really? Can you give a timestamp? Ross Tucker has previously been quite clear on this, so I'm inclined to trust his word, but what you're describing is PAIS.
My understanding is that Semenya has 5-ARD, which would not match that description. The problem is failure to convert testosterone to DHT. From Wikipedia:
Levels of testosterone are elevated, while levels of DHT are significantly decreased, leading to male undervirilization.
The lack of DHT causes problems in genital development, but the testosterone can still largely do its job on the rest of the body.