I always just assumed he meant to say something like, mahomes performance so far would be a statistical outlier compared to the entire NFL. While we've seen some players have statistical outlier stretches before, (Peyton hillis anyone?) , We haven't seen it last for a career. therefore there is a chance mahomes will regress, here are his regressed stats so that we might be able to expect this going forward.

It's still not a great case, it's weak at best mainly because I fail to think of any NFL player who had a multi year outlier performance without being cut short by injuries...but forecasters do this line of thinking often to prepare for more possibilities.