The intersex athletes support this claim because, although their condition is "natural" in your sense, they are in fact being banned from participation in women's sports. For example, the various individuals mentioned above are restricted from participating in distances from 400m to a mile, including in particular 800m! (Thus they have switched to other distances, but the bans are likely to extend there too.)

Let's be honest, that's what most of these bills are marketing themselves on.