Correct, later courses force you to practice earlier skills. This is the mechanism these researchers stumbled upon. Plain spaced repetition explains a lot. SAT is definitely not meant to test performance of learning, though some people with very high SAT scores may be a bit more efficient or know tricks that are not widely taught - SAT is too easy to differentiate them.

Related knowledge strengthens already known things, and once critical maximum stability/retrievability is reached (optimally 7 or so precisely spaced rehearsals) it is pretty much cemented. Without optimal spacing, probably quite a few more repeats. At maximum stability/retrievability the exponential maintains "flat" form for a very long time. A refresher may be needed to get facile again, otherwise it may take a short while to remember and there may be mistakes. Even a trivial refresher will work though, chances to use basic algebra are many.

Principles SuperMemo SM-17 algorithm puts in quantification. Here's a link with all the history and references to some other research: https://www.supermemo.com/en/articles/history

Thanks for the paper, though it's slightly wish washy, but still useful.