With bluetooth headphones I've moved to doing everything I can while exercising. Even sitting down to read a book feels like precious time that could be spent combating a sedentary lifestyle like listening to the book instead.

I find it difficult to do anything intellectual while exercising at a relatively high intensity. What intensity are your workout sessions? But perhaps review is different than trying to absorb a new concept?

Personal anecdote, but there's some optima for information retention at low intensity exercise. This is around moderate to vigorous walking pace or a very slow jog.

Increase intensity beyond that point and your brain's information processing ability drops below baseline.

I've found this consistently true. At lower intensity cardio for example, I can watch educational youtube videos or follow along a movie. At high intensity, I can't even watch an action movie and follow the plot. I have to switch to music and/or a sports action video (no meaningful dialog).

The more brainless the activity, the easier it is to do parallel flashcards. An elliptical or stationary bike session will be easier than an equal intensity run where you have to worry about your gait. Flashcards are little atomic chunks for you to process independent from one another. When you do flashcards the only thing you need to concern yourself with is the short prompt on that particular card in front of you. So even if you are working out pretty hard, you can still work your way through a deck, it's just going to be at a slower rate. This differs from reading a textbook or journal article, where you have to keep a certain amount of the previous passages in memory to understand the current line. You will hit a point where flashcards stop working but the ceiling is higher than with other types of learning activities. This will differ per exercise type, per intensity, and the subject you're learning.