I think this is an overly cynical view. Minecraft is actually fun, particularly with friends. The multiplayer experience is in a niche essentially on its own. This great multiplayer experience encourages people to get enlist their friends to buy the game and play with them.

While I think a $1 billion valuation is probably presumes far too much about the repeatability of Minecraft-like success, I think it's obvious that Mojang's success with Minecraft is attributable to more than just effective marketing. I also don't think it's unreasonable to think that Mojang's highly innovative game design will bring them more success in the future.