> There is nothing in the post you originally replied to implying that anyone is uneducated or an outsider
That's literally what the "poor" in "poor grammar" is doing, though. The phrase itself suggests that the speaker is lesser-than simply by virtue of not speaking in the same way as the listener would.
> If I accidentally type "Eat I bread", it's incorrect grammar even if there happens to be some dialect somewhere where it is valid.
It is not "incorrect". It is different. It is marked, maybe, for many native English speakers. But these are distinct things from "incorrect".
I do not understand how you can say "There might be a group somewhere where who find it valid, but I don't find it valid and therefore it is incorrect." Why is your personal dialect the one that determines correct/incorrect? If you are American, is it incorrect to say "to go on holiday"? If you are British, is it incorrect to say "Microsoft is going to X" instead of "Microsoft are going to X"?
The use of the terms "correct" and "incorrect" suggests that there exists a universal ground truth, and my whole point in this discussion is that no such ground truth exists. If there is no universal ground truth, then there can be no correct/incorrect. The only way to get around this is to establish a specific cultural context where a ground truth can be agreed upon (e.g., a style guide used by a particular publication venue). Otherwise, at best you can only say "I would not phrase it in that way", but you cannot say "that is incorrect."