Spaced repetition turns reading into a physical activity with feedback. If you had an activity that required the knowledge you were hoping to acquire, you would learn at the same rate or faster by practice. Most lessons are encoded really poorly. I think spaced repetition is great, but practice at something is better.

A hacker is just someone who has practiced learning independently and has become exceptionally good at it. The reason people say you can't teach the hacker mindset is because without the underlying drive, there's nothing you can tell anyone. It's like when teachers who lament students don't care what they say so long as they get the right grade, it's because those students are optimizing for approval in a system because that's sufficent for their limited purposes. The more you profess to them, the more you reinforce that learning is passive submission to authority. If you want to make hackers, start with necessity, and technique will emerge as the artifact of navigating constraints. If you want to make people smart, challenge them instead of just telling them things. Hackers aren't defined by knowing more, they're defined by having physically done more. Spaced repetition as it's usually presented optimizes for outcomes in an approval environment that produces people who have been rewarded for cheating themselves out of knowledge and expereince.

I would say, want to learn physics? Build mechanisms or make radios. Number theory? Break cryptosystems. Astronomy and geometry? Sail at night. Lead? Ride horses. Fluid dynamics? Tune engines. Statistics? Write a spam filter. Speak a language? Tell their jokes, etc. Imo, most education is set around meaningless but scalable exercises of professed skills instead of meaningful exercises that are more powerful, but don't scale. We've optimized for scale at the expense of quality. It's the solution to an inferior problem.

So sure, learn spaced repetition, but really, find something and practice it for more joy and better results instead.