Oh come on, they use the kind of stats that many gamers demand themselves. First everyone's bitching that GOG doesn't track playtime and then everyone's scared when devs show that they know some numbers of players who play on Steam or submit highscores.

And honestly: it's not nearly as scary once you've seen it from the other side yourself. It only takes some very basic numbers to be transmitted along with your highscore (for example) and already devs have such stats. Make a game track seconds while it's not paused, transmit the final number along with the stats the player wants to display online and there you have it, reliable numbers on the length of a game session. Then the devs can easily spot the extreme values, determine averages etc., it doesn't mean that anyone's really spying on the players, it's not like they are going through the entire list in detail, determining which nickname belongs to whom etc..

I haven't played the game myself yet but I doubt the devs only use the basic Steam stats for tracking this stuff, though. You get much more helpful results if you make the game itself submit some more internal parameters that are of no use to Steam but are certainly interesting to the devs.