Can you back this up with anything? Choosing how a word is represented when written seems like spelling to me.
It’s grammar in the sense that it encodes functional information: the distinction between “persons”, “person’s”, and "persons’ " is a grammatical one, the same way as writing “person” when you meant “persons” is a grammatical error rather than a spelling error. Writing “parson” when you meant “person” is a spelling error. (Any of the above, of course, might be a typographical error.)
Discussing grammar niceties in this manner reminds me of that lovely equine aphorism:
You can lead a horticulture, but you can’t make her think.