It is a bit more challenging, but it is workable.

Same caveat as in the article: Spaced repetition is just one (minor) part of learning math/physics. It alone won't get you anywhere.

For math - particularly higher level math, the most obvious use case is definitions. There are so many!

You can put theorems in there, but it is a bit challenging on how to phrase it. A single theorem could result in several smaller flash cards.

I think what works better is taking a theorem, finding a representative problem that is solved via that theorem, and make the problem statement the question. The downside of this approach is each card takes longer to process as this is not just plain recall, but actively solving a problem. For this reason, I keep such cards in a separate deck and review them only when I have time I can dedicate (e.g. spending well over a minute per card).