"They/Them" would be appropriate if we didn't know the person's gender or if they were specifically non-binary. However, in this case, we do know that this specific person is "he/him" thanks to the username and public profile, so the comment parent to yours was appropriate and your comment less so.

(Your comment is less appropriate because the analogy would be if you said: "Nice to meet you, Jim. Not all people in your position are men, though.")