Ok then please explain why Socrates is better than margin notes for an assumed purpose, because that is not reasonable common knowledge.
If by that you meant to suggest that for formal study, some kind of self-review process combined with cold written practice/rehearsal sustained under interleaved spacing, then that is likely the gold standard for comprehension and memorization. But most people don't even know that, either.
But the main premise is we should not assume all book reading is for rigorous self study. Margin notes can be useful for other purposes than direct understanding/retention (I admittedly don't know the psychology evidence for it but many academics routinely write margin notes here and there on (poorly written/edited) papers for purposes of idea generation, scientific criticism, etc. Presumably the Edison example mentioned above would have been more like that.).
You are correct that the reference is a little obscure -- if my memory serves it's Plato's Phaedrus.
Plato argues, in a book, in the voice of his mentor Socrates, that books will destroy knowledge. And the reason is precisely to do with the thing that Socrates is famous for, his Socratic Method of asking his students (or rival politicians or anyone) questions about what they claimed to know, engaging them in a real discussion where we come to things that we agree about and then build that agreed knowledge-base incrementally. For Socrates, a book could never provide that experience; you cannot ask the book a question.