He says that spaced repetition is effective, but basic repetition of re-reading, re-watching, re-listening is not effective.
Spaced repetition with "active recall" comes out significantly more effective - instead of exposing yourself to the same material over and over, challenge yourself to recall the material at the time when you're on the edge of forgetting it; the active mental effort of doing that appears to fix information in memory much more effectively than reading or hearing it again.
A consequence of that is his suggestion that notes and review material should not be facts you want to remember, but questions that will prompt you to think and recall what you want to remember. "Writing questions for yourself makes you engage in cognitive effort, and the more brainpower it takes to recall a fact, the better strengthened that connection seems to get, according to the evidence at least".