There are very few cases where mods edit comments, in almost all cases deletion is the better option. And if a mod edits a comment, they typically only remove parts of it or replace offensive terms with less problematic ones. Those kind of edits don't really need any protection by the CC license, I wouldn't count them as a proper contribution.
From the Creative Commons FAQ:
In order for an adaptation to be protected by copyright, most national laws require the creator of the adaptation to add original expression to the pre-existing work.
The section on when modifications must be indicated:
In the 3.0 suite, the obligation to indicate if modifications have been made applies if they result in the creation of an adaptation (when allowed by the license).
A moderator should only correct small mistakes or remove content from a comment, they should never add anything substantial. For that reason it is extremely unlikely that a moderator edit would meet the threshold of originality to be considered an adaptation of the original CC-licenced comment.
We moderators also know that our edits won't be attributed to us, so if we had a problem with that we could just stop editing comments.