With this recent implementation, does it also mean that any plagiarism that isn't detected with the space of 6 months can go unpunished?
I don't think so, plagiarism will be handled specifically by moderators so you can report the case even if you find they did it more than 6 months ago. The same is true if you find malware or something harmful to forum and users, as Halab said.
The warning above is very clear that moderators do not moderate old posts for any reason.
No, that's not exactly what it is written. If there is a
very good reason to report an old message (malware, doxing, etc...), report it and a mod will handle it (normally).