If they were actually impressed by you, they would've probably hired you. If you can come up with a 3X MAU strategy in a single interview, then of course they'd want to hire you. Imagine what you could do in a single month!
Chances are some other part of the interview went wrong in a way you didn't recognize, and perhaps the same idea also was raised internally or by another candidate.
Not everything suspicious has a bad explanation. Sometimes reality is simpler.