At some point in our education we have to memorize the rules for manipulating algebraic formulas, trigonometric identities, etc. It would be hard to learn calculus without being able to do these things unconsciously.

On the other hand, what a professional mathematician does is create new tools for proving theorems. This can't be memorized because, by definition, the knowledge didn't exist. It's true that they draw on previous experience having solved similar problems. But it's not clear to me that this knowledge is most efficiently gotten by drilling the same problems over and over.

I think this idea of using Anki for learning everything is misguided because it attempts to reduce complex topics and pragmatic know-how to a set of trivia questions. An example from the article: the author wanted to understand the AlphaGo paper, and some example flashcards were "who plays first in Go" and "where did AlphaGo get its training data". They probably could have acquired a much deeper understanding by playing a few games of Go and implementing a reinforcement learning algorithm for tic-tac-toe. Incidentally, that also sounds a lot more fun and motivating to me than studying flashcards.