I don't think anyone is saying to purposely slow down to protect their defenses. I think the case is trying to be made that going fast can be a detriment to your defense in a game. If your defense was just on the field for 5 minutes, you get the ball back and then go 3 and out in 45 seconds and the defense has to come right back out, it starts to add up. I'm sure there is some sort of statistical correlation that would back this up, I'm honestly just to lazy to look it up lol. Going up tempo can be a huge benefit when it is successful but if it's not working, you are definitely putting your defense in a spot to be less successful.