No, that's not obvious at all. A single event is evidence that some abuse still happens, it does not tell us how much more abuse there would be in the counterfactual where the history wasn't available.
I get what you're saying, but I feel like they should highlight comments in some way if a repo admin completely replaces a comment with different text. I'm struggling to imagine a situation where that would really be appropriate. The "Edited by: username" seems too easy to overlook.
discourage != prevent all