Do you know any small indie games that succeeded because of an amazing marketing strategy and NOT because of word-of-mouth/social media and the fact that their game is really, really fun?

I think you're being a tad critical.

In all fairness, having a really fun game IS more important than anything else. Definitely more important than marketing at this stage, where the game is not yet developed.

It's hard to make money marketing a game that doesn't exist.