the DBs had to cover the receivers. I'm sure there is some code that they yell when it's a punt to get players away from the ball, but Johnson would have had to hear that, break his coverage, and then figure out where the ball is and where it's going all in the second a tiny punt takes.

It's a fluke play. It would have been nice if Robinson was able to recover it, but he was coming in at speed and had to make an adjustment on the ball bouncing quickly. It was a heads-up play for him to recognize it went off Johnson's helmet to even try to recover in the first place. But if the punt is 6 inches shorter, it hits the Kansas player and BYU maintains possession. If it's 6 inches longer, it goes over their head and bounces into the endzone for a touchback.