First off, that's not really "harasment", it's insulting, yes, but harassment would require multiple acts, not just one comment on a post that was deleted half a month prior. You specifically had to seek out that comment in order to see it.

And the reason they probably don't want to deal with older posts is because it'd mean they'd have to either do math to deal with when the neutral was added and when the offense took place or have the neutral end an additional 2.5 months after where it would have if they gave the neutral when the comment was actually made.