Spell checkers are often terrible, and are just as capable of introducing errors as fixing them, and don't always catch grammar errors like lets vs let's.

Look, I care a great deal about proper spelling and grammar, and put a lot of effort into it, but errors can still happen (I've noticed I've been making more spelling errors in recent years than I used to, which worries me). Your comment just sounds needlessly dismissive of people with dyslexia. You could also put effort into proper spelling without trying to come off as a dick (which frankly is probably worse than bad spelling).