"Not living up to the hype" and "flopping" aren't the same thing. Fable was hugely successful sales-wise, and garnered respectable reviews. Just because Molyneux spent years hyping it as the best game of all time doesn't change any of that.
Personally, I've never understood people's obsession with marketing hype. I really don't care how much a developer hypes their game; in the end, it doesn't affect my gaming experience in the slightest. The quality of the end product is all that matters.