@baldur From my own experience working with the classic spelling and grammar checkers (I open-sourced it even, subtitle-linter on my GitHub) you are overselling them.

Once I even got into an internet fight because I said spelling checkers should reject rare words, as, in my experience, most instances of rare words in the texts I reviewed were typos — those were Portuguese texts, but if I were to give an English example it would be like finding lots of "fain" as typos for "pain" — in Portuguese there were LOTS of "maça" (mace) for "maçã" (apple).

But, to be fair, even LLMs have issues with this due to how they are designed. On the other hand, it's more likely a LLM would recognise mace if it's being held by a king, but wrong if someone is eating it. And all of this is just spelling!