The point is that (1) most people are in fact going to slack off for at least that much time, and (2) doing so in a controlled way is actually beneficial to you

The point is that (1) most people are in fact going to slack off for at least that much time, and (2) doing so in a controlled way is actually beneficial to your performance when not slacking off. The only evidence I know for these is anecdotal, but I'll take a substantial body of anecdotal evidence over "sounds a bit too much to me" any day.