If someone is insensitive and disparaging, you can only tell that they are insensitive and disparaging, not that their arguments are wrong or that they are mistaken about some factual matter.

To think otherwise is the "ad hominem" fallacy.

One might generously think that Ms. Millen is so incensed by the situation that she's taking it out on Thomas in a mean-spirited way. One might ungenerously think that she's a virulent transphobe. Neither of those things would make her wrong--but they might make her biased which means we should check her more carefully not assume she's wrong.