As far as I see, PhD isn't really about studying or schooling - unlike all the previous school programmes, it's essentially an "apprentice researcher" job, wher

As far as I see, PhD isn't really about studying or schooling - unlike all the previous school programmes, it's essentially an "apprentice researcher" job, where you work on research (possibly studying whatever you need for it), you get given some supervision by a more experienced researcher and once you demonstrate that you're capable of independent research by doing some and writing it up, you get a degree certificate. And it's not necessarily about some institution staking its reputation on it, the main evaluation of your research would be by external peer reviewers looking at your publications, not by your institution alone. A PhD is not required to do research, but research is (or should be) required to get a PhD, and research done outside an university program would generally be sufficient to get a PhD - there are paths to get a PhD degree mostly based on existing publications or based on a 'monograph' such as the research-based book that the OP has written. Learning and schooling alone, on the other hand, is not sufficient for a proper PhD; if you've learned everything that's known and published on some topic, that's not enough, you need to make at least some novel contribution.