The researcher had 16 different adults use the game as directed, and had them performing with a high heart rate and oxygen consumption, burning a minimum of 200 calories a session, five days a week. According to the ACSM, that's enough to "maintain proper health."
Unfortunately, the game doesn't actually stop you from playing Red Dead Redemption and connect the monitors up to you, so you'll have to do that part yourself. And while 200 calories can be significant (depending on your diet), you can burn just about that much by walking for an hour. Then again, this "walking" game doesn't have any DLC coming, does it? Take that, ambulation!