Last post on this I swear. One thing that I should have mentioned: while the z-scores do adjust for the balance of the league, they don't adjust for the overall quality. Perhaps there is more balance because the good teams got worse, not because the bad teams got better.
We can attempt to answer that question by looking at the EPL's performance in Europe. If English clubs performed relatively poorly in Europe, we could conclude that the quality of the EPL was low in that season and if they performed strongly, we could conclude the quality of the EPL was high (this of course assumes there is no worldwide drop in talent, which would seem to be unlikely in the extreme).
Fortunately, UEFA has a statistic used to judge every league's performance in Europe, called the UEFA coefficient. This year, the EPL's coefficient was the highest it has been in 5 years at 18.357. This, combined with a very balanced league, means that the EPL was stacked this year and that United were the best team of the lot. Again, all the evidence points to United being a very strong team this year, contrary to the popular narrative.