This article isn't about creating a repository with non-standard properties I'm afraid. It's just a set of instructions for copying one repository into another. The resulting repo isn't any different from any other Github repository and doesn't have any kind of 'mirror' status.
https://docs.github.com/en/github/creating-cloning-and-archi...
I'm not sure if a mirror repo actually blocks pull requests. Check with GitHub Support for more information:
[email protected]