The interdict of Merlin stops the indiscriminate spread of high level magic, for a definition of high-level that is relative to the learner. It is probably possible in principle to write down a series of hints that lead the reader to figure out enough of the concepts themselves that they can read the next level of hints. I expect lots of people *try* writing down their material and leaving such a trail of hints, and while it usually fails Riddle probably independently figured out enough stuff to bridge the gaps.
The interdict of Merlin stops the indiscriminate spread of high level magic, for a definition of high-level that is relative to the learner. It is probably possible in principle to write down a series of hints that lead the reader to figure out enough of the concepts themselves that they can read the next level of hints. I expect lots of people *try* writing down their material and leaving such a trail of hints, and while it usually fails Riddle probably independently figured out enough stuff to bridge the gaps.