I think the point is that you could occasionally read thru your relevant (for whatever interest you have) notes to keep those ideas ~refresh in memory. I doubt anyone's advocating for reading every note you've taken in school 20 years ago.
AFAIK our brains are not saturated SD cards, learning something doesn't remove an older file.
I inferred from the comment I was replying to that the commenter was disappointed to have forgotten this particular information. You can’t regret forgetting something while also considering it irrelevant garbage.
I read it as him pointing out that taking notes only helped him while the information was immediately useful and at this point he's forgotten it as thoroughly as if he hadn't taken notes at all.