I don't think he'd assault the person either, but you're missing the point either intentionally or otherwise.
The fact that he is emboldened to say that - and you embolden him actively to say that by arguing not against the comment but against the people who call the comment out - is a big fucking problem whether he'd actually have the balls to try to attack someone or not. It's easy as a cis white straight man to be like "lol he wouldn't ACTUALLY do it" but put yourself in the mind of a trans person. There are dozens of millions of people saying shit like that every single day in this country. How excited are you to take a chance that the one behind you in the bathroom line is one of the ones who's just bloviating? Don't you see how wildly dehumanizing and degrading it would be to see nearly every public discussion about your identity go that way? And to see people who to your face call themselves "allies" defending the comment (and yes, saying 'hes just SAYING he'll assault them, he probably wouldn't really DO it' is defending it), and instead going after the people who call it out?
If I said "I'll kill any white person who enters the bathroom after my daughter", that's a problem even if I don't literally murder them. If you have that thought, there is basically zero chance of someone - especially from the other side of the political aisle - convincing you bigotry is wrong. You already know, nominally, that bigotry is wrong - but engage in it anyway.