Emphasis on 'potentially'. Don't waste too much time on them, some of them get off on watching you uselessly spend your time and energy and sanity on them. Brainwashing doesn't work like it does in comic books. These are people who were already willing to and capable of rationalizing putting kids into cages and the 'brainwashing' just gave them an easy explanation.
It's like a person who has committed rape. Even if they come out later as an anti-rape activist and they're publicly remorseful and they do a lot of work to make up for the harm they caused you still aren't gonna leave them alone with your sister.
I think we should consider this the way we would any other form of hurting people. So long as you don't have to, it's wrong to do so. I agree that we shouldn't body shame and I question the need to do so even in this instance, but I have to acknowledge that those on the right do not respond to reason or appeals to their better nature. If by mocking them we can hurt their ego enough that they start acting better to stop the mocking and that stops them from literally killing people then by calling them 'short' we saved lives. War is messy business and I'd much rather hurt someone's ego and cause the societal harm of normalizing shaming about body type than have to kill somebody.
I'll reiterate that body shaming is an ugly thing and the only way to justify it is as the lesser of two evils. We have to hate fascists openly and unabashedly. They have to know that all the social niceties we give to tolerant people cannot apply to them.
Body acceptance is a form of tolerance and tolerance is a social contract. It's dangerous to body shame but not more dangerous than tolerating the intolerant.