Duke comes in on passing downs where defenders are playing back and willing to give up chunk yards as long as they prevent a first down. Duke has only faced a stacked box (8 or more defenders in the box) on 9.4% of his runs. For comparison, the rates for other top RBs - Breida (32.7%), Saquon (32.4%), CMC (21.9%), Nick Chubb (17%).

It's just as bad when you compare his light box rate (6 or fewer defenders in the box) vs what other RBs face.

There are definitions of what a "successful" play is. For example, a run of 10 yards on 3rd down that doesn't get a first down is not a successful play. Defenses don't care about the yards as long as they prevent the offense from making successful plays. There are advanced metrics that take into consideration if a player makes a successful play or not. Duke Johnson's rating for that metric is negative which means he's much of his yards gained are on unsuccessful plays.

So his super high yards per carry is basically smoke and mirrors. An illusion that breaks down as soon as you take a closer look.