More likely a sign of his ego and desire not to have to do all the running, DM is not a sexy position. I cant remember the quote now, think it was Redknapp who said all the worst young players claim they are a number 10, or behind the striker
Nothing to do with running, he wanted to play as box to box role (Emery implied that) and that involves lot more running than playing a disciplined DM role.
As per Athletic, he covered 2nd most distance in Juve side, also Emery said " When you want to play with a defensive, creative and box-to-box-midfielders in fixed positions, Rabiot finds himself confronted to a problem. He has to run, run, run and not play in a fixed position statically. And even less so with his back turned on the action. "
which says more about him wanting to run and run rather than be a static DM.