While the author probably wants to own the main code, I'm sure there are other things he may want to get help from external contributors.

For example unit tests. At least to show that it works, and how to use it. Also a build config, like gradle/maven, so others would be able to use this lib.

The author says that: "Reviews, tests and comments are welcome"

I thought github let you turn off PRs on a repo -- the author may wanna do that.

No, you can't disable the PRs tab; you can disable any tab except Code and PRs. Torvalds famously complained about this because he handles pull requests through email, so all PRs on his Linux repo are useless.

PRs cannot be disabled.

...without archiving the repo.

Archived repos however have that ugly yellow warning.

Can you still push to the repo? Perhaps via --force?