They do not have sales data, so they use two different proxies: number of reviews, and number of active players.

I don’t see mention of how they get the number of active players. I’m assuming it’s through stats in Steam or something similar. If that’s the case, then their assumption of this number being biased towards being larger than the true number would be wrong. If you choose to both pirate and buy the game, chances are good that you’ll be playing the pirated version, and therefore would not get counted towards active players.