I agree repetition is key. But here's what I noticed from personal experience. When I first read something interesting, it might not stick.

When reading it again (say: the same book a year later), knowing I must have read this very interesting thing before, but apparently forgot I more or less have the same reaction as being SURPRISED. And more often than not THAT is when it really sticks.

This is from anecdotal, personal experience, but being surprised sort of unlocks things in the brain that make things stick (I could go on, on how this would be a biological advantage etc. but that is reaching) and I guess that is what Sivers is pointing at?