perpetuating the false and transphobic stereotype of transgender women being men trying to assault women.

That is not at all what was said or what was being implied...

Sports can be dangerous in and of themselves, which is why there is an entire field of sports medicine, as well as safety guildlines for just about every professional and amateur sports league that exists.

It's not unreasonable to think that a circumstance could arise where two people of the same gender identity fall into radically different body weight/height classes. And in that circumstance it would not be safe or fair for those to people to compete. (This is why boxing, for example, is already broken up into weight classes.)