Sorry, guess I'm kind of contrarian - reading a claim like "spaced repetition is the best way for learning facts. And it's not just my fancy - here's one of the many studies that prove it" just makes me think "Uh, that sounds highly unlikely" and download the paper and read it. (I mostly studied philosophy, so probably that's more pedantic about accurate language than most humanities areas, or should be.)

Anyway.. So, unsurprisingly, it doesn't prove spaced repetition is the best way. It talks about another paper that looked at 10 methods, and found only 2 were 'highly effective'[0], spaced repetition and practice tests. Even if spaced repetition had been the single best way, and they had tried every possible learning method, not just 10 methods, that would be nowhere near proving it. (The next study might find something entirely different, as continually happens in science.)

So, sorry if that seemed uselessly pedantic. p.s. I do use Anki though. Or I did. Not sure how useful it is, although better than no study, if that's the alternative. I used an ancient Windows program years ago for learning Spanish, which was similar, but also had audio of a Spanish woman saying the words to go with the written word; I seemed to learn more from that.

[0] The 'utility' scale in the other paper seemed to have only 3 degrees - low, moderate, high.