You mean figuring out answers to important business issues like "who are you users?", "how will you acquire them?", and "how will you make money?" and then having a plan how to clearly communicate them is cramming and nothing more?
By doing flash cards, they were able to accomplish the exact goal you stated much quicker and more effectively than blundering their way through the YC interview and trying again in 6 months after they've had real pitches. Or should everyone just be as amazing as you are and be able to communicate these things without ample practice? Ridiculous.
They test for how prepared you are and how motivated you are in working to be given an offer.
Asking "what the interview tests for" is a pertinent one as any entrance process becomes increasingly formulaic.