I think criticizing people for what they do (eg: calling some politicians “murderers” because they allow genocide) is different than calling names just to hurt, which is what dead naming usually is.
I don’t think anyone should be okay with racism or that kind of attacks, even on people they hate. Kim Jong Un may be a piece of shit (or not), but calling him a racial slur is unproductive and definitely crossing the line.
Here, the deadnaming comes from ignorance, not hate; and it’s true that it might help clarify who someone is, but in this case everybody knows her for being the “leave Britney alone” person, not for her deadname. So it’s really unnecessary.
The funny thing with gendered languages is that synonyms can have different genders. So “el pollo” and “la gallina” both mean “chicken”, but their grammatical gender differs.