They barely mention why they're even doing this. I see flashcard as a means to something (e.g. learning a language, preparing for a test...). The measured outcome should be the success in task, rather than the number of reviewed cards.
I see no reason why that is important.
For example, maybe I read a book that has a really key insight about how my brain works with ADHD. I’m not going to remember or apply that insight to my life unless I spend some time trying to remember it. By default it’s not going to stick for very long.