I think you are overweighting a fairly simple grammatical error. The commenter expresses themself clearly and logically. It is possible that they don't speak English as a first language, or that they simply are not that careful about making grammatical mistakes. Not everyone is as pedantic about language as you or I may be.
Unreasonable people can write grammatically, and reasonable people can write ungrammatically. I think it is better to judge an argument by its reasonableness.
I think many people would consider your response impolite and unkind to the original poster. Surely you do not want to shame someone for their lack of mastery with the English language? Surely you would rather judge an argument on its merits?
May I suggest a last question: could you see yourself reading this comment to the original poster face-to-face? Does it not seem rude and condescending to imagine yourself doing that?