A side-bar article to the one Tony links shows that the average winning score for PGA Tour events was actually lower (i.e. more under par) than the average score at European Tour events.
But that stat might be as irrelevant as Rory's cherry-picking of just a few high profile European events and then (implicitly & unfairly) contrasting them to high profile events/majors in the U.S.
All of which is to say: I think Rory -- who one can't help but like, as a wonderful player and very decent lad -- is annoyed about something, but I'm not sure about what.
Maybe his complaint is a young top golfer's equivalent to an old great golfer like Jack N advocating for a roll-back, i.e. his way of saying that technology (and course set ups/designs) are artificially narrowing the gap between the very best players and all the rest of the tour pros -- with Rory, naturally, identifying himself with the former.