I don't know where they got the 15 hours from. Perhaps you can do it if that is all you focus on, but I think you will need at least 20 hours, probably a bit more. (my Steam says 50 hours, but that includes quite a bit of time where the game was running but I wasn't near the screen)
Also, someone noted that there is no tutorial. I haven't tried the demo (of course) but the full game has a very decent tutorial.