It's not correctly adjusted for one. Look at SAF's last season, scoring barely higher than Mourinho, LvG and Moyes. Looking at the Moyes season, our 2 highest scorers have 154 minutes between them. If you only count players with 500 minutes and more the average goes down a few points whilst SAF's season goes up a few but then you're still counting a player with 600 minutes equally as much as someone that played 30+ games which isn't comparable.
Then you have to factor how the rating is amassed. Does LvG's sheer number of passes yield a higher average rating for players compared to SAF's direct and more risky football or is it passing success that counts irrespective of the number of passes?
Another note, City's squad last season average a 7,24 (7.19 if you count only 500+ minute players) which isn't a whole lot more than the two high scoring SAF seasons (where he didn't win the PL nota bene) and yet we clearly know that this City side was comfortably a better performing side.