The PR is not in their repository, it's in the fork. Pull Request refers to "pull this from my repository, here's a link", GitHub just presents it in a convenient interface.
(But you're right, those who send DMCAs are likely to just a send a link to the original repository :)
One can assuredly delete the pull request from your own list of open pull requests, right? It isn't that the information exists in theory on GitHub: it is that it is still listed right there when you go to check the health of the project's open issues/PRs.
