Even if I agree that it wasn't really necessary to clean up an old topic, these multiple posts have probably been reported. Who is to blame ? The mod or the person who reported them ?
Anyone can report anything, we can't blame them.
If the mod hadn't done anything, we would have seen messages saying: "it's a scandal, my reports have not been handled or marked as bad !!!"
I don't think mprep will base his moderation on possible complaints about not deleting reported posts.
And from what I can see in the topic you linked, the multiple posts of several members have been merged into a single post.
So, is it really a big deal ?
I'd say it's not necessary, and even if posts aren't deleted, merging them can break other links (such as Reference links in Trust ratings).