I think it is the opposite. The EPL is so competitive week in, week out and there are few walks in the park - this leaves English teams gassed in the latter stages of European competition. Meanwhile, in most years, Germany, Spain, France and Italy - have 1 or 2 teams that dominate and aren't worn down as much in their own league play.
I do think the fact that teams like Brighton, Newcastle, Villa, West Ham, Spurs, Wolves can all scalp the best teams does speak to the overall strength of the league. That is tempered somewhat by the fact that United and Chelsea are much weaker than they have been in the past.