Certified by the person who wrote the library? I guess I also certify it
Self-certification doesn't mean that the process lacks validity or rigor. On the contrary, it involves thorough testing and validation against industry standards to ensure that the software meets the necessary criteria for interoperability, security, and functionality.
If you're curious about the specifics of the process, you can find more information on the OIDC certification FAQ pages. These resources provide detailed explanations of the certification process, the criteria for certification, and the testing procedures involved.
In any case Filip Skokan has essentially made a career out of building open source OAuth stuff, so even if it's a bit humorous that he certifies his own stuff, it's likely that this implementation is one of the most compliant out there.