Precisely. Plagiarism is a dangerous "mistake", but I also have a little doubt. Despite the fact of the act itself, meaning that any plagiarism is quite terrible, for you are stealing other's work, can the forum get into trouble for the plagiarized content in here?
Pliagarism is not a 'mistake' but a violation of the rules by users who are looking to get credits (and possibly merits) for works of others or those who wants to boost their post count without thinking up what to post.
I have not heard of this new regulations, would you mind linking it?
As far as I know, a forum is not directly responsible for the contents on its platform.