TL;DR: Edits on old posts are encouraged, truly minor edits should be dismissed with "no improvement whatsoever".
This is the official policy and the reason why said rejection reason exists. As for why editing old posts is technically encouraged, well, there are badges for doing it (bronze, silver).
However, if the edit actually (somewhat) improved readability and you maybe see a meta reason (eg it is unanswered) to bump it, then I'd usually be inclined to stretch the above policy a bit. An example for rejection-worthy edits would be changing things from "English (Simplified)" to "English (Traditional)" or adding a tag that won't help google-discoverability (e.g. where the tag name is literally in the post body or title).
If you find such truly rejection-worthy edits to be happening a lot from a specific user, then I suppose you can ping a mod in chat (sadly, you can't flag user profiles :( ) and we'll communicate it to the user to tone down the pointless editing a bit. Also I think if enough edits are rejected, then an automatic edit-suggestion-ban should kick in.
For further reading, also check our help center on the subject:
Why can people edit my posts? How does editing work?