Imagine this scenario:
1) I make a copy-pasta 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.
An edited and changed plagiarized post that points to the correct source is no longer a plagiarism issue if you ask me. What you explained can happen to anyone. I don't think the timeframe for how long the content was on the forum should be decisive. If the post was edited before someone reported you, I think you wouldn't be banned. Even if you edit the post after the report is sent, the admins might look the other way.
In the example you provided, the admins can't prove (unless they have some tools that I don't know about) how long the post was there without the source. Therefore the original achieve doesn't offer any clarity or additional evidence.