Saying that truth should serve life is the converse of the naturalistic fallacy. We can say that if something's natural or factual, then it's automatically good (or bad). That's the naturalistic fallacy. Or we can say that if something's good, then it's true/factual/natural. That's a kind of pragmatic appeal to beneficial consequences. Religious folks do that when they say we should believe that God exists because religion makes us happy.

Both moves can be quite fallacious, and Peterson likely makes both of them at different times.