- This is definitely not spam. But could it be considered a bit too much "affection" for Wikipedia by adding links that might not be relevant
Addition of links wasn't sole purpose of edition, he fixed grammar; they're rather bonus, not hurting the post, so if the editor likes it, why not?
- Are those links actually relevant? Do they improve the post?
In the particular case it's subjective (anybody not knowing HP here?). So, I'd approach from the opposite side: they doesn't harm the post.
- Should these kinds of edits be: 1. accepted without caring too much about them? 2. encouraged because they provide useful background? 3. rather rejected to avoid cluttering the post?
That depends. If the linked edit would've made for sole purpose of adding links, I'd rather reject it, because it would add from little to no info to the post, and trivial edits are discouraged.
Another reject situation as if a post already has important links, like to bug-reports, blog posts, tutorials one used, etc; and the edition clutters available ones with those that didn't help much. As an upper bound situation imagine a post with every word linked to a dictionary.
In other cases, i.e. when edit was made for purposes of fixing (like grammar, formatting, etc), and links doesn't hurt to readability, it is fine to accept.
As for the fact that links are made specifically to Wikipedia — I doesn't see it as anything bad at all: in the end, it is an encyclopedia, the more that it is a non-commercial one.