I believe revenue would be earned through in-game purchases to...

I believe revenue would be earned through in-game purchases to help the player progress and "level up".
If you watch the gameplay that was posted on the first page of this thread you can see that your character has coins that can be spend of weapons, tools etc. I assume in game purchases can be made to get more coins.
The game itself is free which I personally believe is better because more people are happy to download a free game to just give it a go... then they get hooked.

All imo, I could be wrong!