My interpretation of “we don’t know” is that the space of possible instruction methods is huge, and we don’t know what works best. It is easy to think of it as a kind of optimization problem, but that line of thinking quickly runs into a bunch of practical obstacles: different people learn differently, studies with good experimental design take time and resources so it is slow to iterate, and good teaching seems to require deep insights into student motivations and incentives, aspects of teaching that are hard to quantify. We also don’t have great ways to measure outcomes; grades are weakly predictive of future success.

The best teachers seem to combine deep thinking about how to teach with intuition and charisma. The ingredients are hard to replicate.

I do think that this is a nut that we will eventually crack, but it is proving to be a very tough nut.

It's also quite possible that the method that works best for one person is completely horrible for another person, but that's just me guessing