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 ?
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 !!!"
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 would blame both the reporter and the mod that removed the posts. It is the mods job to make sure the action they are taking conforms with the rules, otherwise what is the point of having him as a mod?
In the thread I linked, when the posts were made, they conformed with the current implementation of the rule (that wasn’t in effect at the time). The person who I was interacting with had deleted his posts in an effort to cover up his fraud, but that is a separate issue.
Frankly, pretty much any report on posts that are 5 years old should be marked as bad. There is no reason why action should be taken on posts this old in nearly all cases. If you are going back this far to report posts, you are either trying to create trouble or are trying to pad your stats.