I would suggest that it's not necessary to speculate too much about why they made a grammatical error. There are many possible reasons. For instance, I suggested that they may simply be less pedantic/careful about grammar. They may simply care about the form of their expression less than you or I do.

I think judging a hypothesis by its form/expression is not a great way to get at the truth. If a heuristic has to be used, then probably tone, coherence, and even-handedness are better than grammatical correctness. Those are at least closer to the substance of the argument.

I suggest that evaluating arguments on the basis of form/expression will not help you get at the truth.

It is your choice whether to be aesthetically dissatisfied by grammatically incorrect English. Many would consider that pedantic, though I might have a modicum of sympathy for you. However, I think the error you've made is to promote aesthetic displeasure into distrust for the OP's reasonableness.

I do not know about others, but I do not think I am losing my mind about anything. I suspect that most direct and unapologetic people have faith in the substance of their arguments, and would be frustrated to be judged using low-signal heuristics like grammatical correctness.