Let's pretend for a moment that we know that Reddit has any sort of decent versioning system, and that it keeps the old versions of your comments alongside the newer ones, and that it's feeding the LLM with the old version. (Does it? I have my doubts, given that Reddit Inc. isn't exactly competent.)
Even then, I think that it's sensible to use this tool, to scorch the earth and discourage other human users from adding their own content to that platform. It still means less data for Google to say "it's a bunch of users, who cares about the intellectual property of those filthy things? Their data is now my data. Feed it ~~to the wolves~~ to Gemini".