If they don't outright assault anyone and are just being a creep, people can use the fake trans argument to get away with it. Potentially. I don't think a lot of people would buy in to it, but there are conpeople who make conning into a lifestyle, so it's not entirely unthinkable a malicious person could sneak into the wrong room to ogle people, get caught and then launch a twitter flame war ruining the life of their victim by saying it's transphobic not to allow them in. Theoretically. I don't think it's likely, but there are enough people in the world that unlikely is still worrisome.
(Heck, I'd advocate for separate locker rooms for everyone individually just because being naked is really uncomfortable and many people don't want to be seen by anyone.)
There was a controversy about a person saying she was a trans woman, but who still had the male physical parts, wanting to get their privates waxed by people who were not comfortable with waxing male parts. When they refused to do it, the person launched a social media rampage that was a mess for everyone involved. I'd link an article, but I can't remember any specifics. I think it was in Canada? I'll see if I can find it.
When it came down to it, that person just made things harder for trans people who do want to respect people's boundaries and acknowledge people aren't comfortable with touching all types of private parts.
So it is important to at least keep talking about these worries, as unfounded as they may seem, because something can always slip through the cracks of what we believe to be a perfect solution.
(I wouldn't say JK is handling it well though)