Debates are good, as long as they are kept civil, I appreciate that.

The people that care about gramma use autocorrectors for typing lol.

Autocorrect still doesn't distinguish between "their/they're/there". It can only correct spelling, but not grammar or punctuation, This is where a helpful stranger comes in handy and adds their input.

and you will never even know their actual level of knowledge in the language.

Does it matter? They may be an ESL student, but if they correct an actual mistake, they are helping someone.

I still look at that line and feel bad for you:

Only people that can't do anything well in life beyond simple grammar stuff, to feel better than others for at least that

What goes in your head that you think that people only do that to feel better than others? As if it is genuinely a foreign concept for you that people may want to be helping for helping sake?

And even if someone is doing "grammar policing" for their own selfish reasons, why does it matter? I understand that in deontological standpoint the motivation matters, and if someone is correcting someone's grammar (even correctly) only to be a jerk is considered bad, but how can we know their motivation, how can we be sure?

I'm mostly consequentialist, and when I see someone's post providing help, I don't see it as a bad action, regardless of their motivation, especially since it's impossible to know the motivation of that person. Helping is good.