Of course they can modify the archived posts at any time. I'm quite sure LoyceV has removed or edited some doxing attempts once or twice. But they're staking their reputation on it and we can trust that they wouldn't do it for malicious purposes about as much as we trust them not to scam or otherwise deceive in general.

(the following is not necessarily in response to you but on the topic of the thread in general)

I do trust both TryNinja and LoyceV but I would still argue that moderators should not use those archives to ban users, particularly if the ban reason itself no longer exists in this forum (the post has been edited by the author).

Imagine this scenario:

1) I make a post, forget to add a link, edit the post 10 seconds later and add the link. LoyceV's archive has my original without the link.
2) I edit the post a year later for an unrelated reason. Some shithead finds the archive and reports it for plagiarism. My last edit date makes it look like I made the edit recently and the plagiarism existed for a year. Archives don't really capture every edit to show what actually happened.

Do we really want to go that far into the weeds? For what purpose? To catch one or two more plagiarists? There is always many others we can find with far less effort. And if we ever get to the point where the few sleazeballs who edited their posts after being reported are the only shitposters on this forum, I think we'll be fine.