Do you have a source for this? I remember people talking about it being possibile for them to do (and now redact.dev even replaces old comments with randomly-generated sentences) but I don't recall them being caught actually doing it.
It's a 'misunderstanding' of how reddit works. It turns out that you can't delete comments from subreddits that are private without the required posting permission flag.
So when subreddits went 'dark' during the protest, and then people deleted their comment history, the messages on the dark subreddits came back when the protest ended and the subreddit became public again. At that point you can just delete the comments again of course, unless you deleted your account, in which case those necromanced comments stay undead forever.
An 'unfortunate bug' with deleting comments they told me when I asked them about my full comment history (edited to be empty and subsequently deleted) being restored. Many of those subreddits did not participate in the protests.