Regarding the whole "harder to dominate" bit. He's right, it was. Today, the greatest players are the greatest anywhere. Back then, surfaces were polarizing, and being hot stuff at RG didn't mean you were a favorite to win Wimbledon. It is what it is.
At the same time, I highly doubt Sampras would have been nearly as dominant in this era as any one of Fed, Nadal or Djokovic. Ignoring the dynamics of his playing style even. Sampras just wasn't as consistent as any of these guys. He lost to dudes he had no business losing to, even on surfaces that favored him. He had his off days. He had distinct weaknesses. Things that subdued his dominance independent of his competition.
Furthermore, there's a lot of subjection that goes into what constitutes who's era, etc., but taken by decade, the numerous regarding Major winners in the 90's and in the 2000's are largely similar:
Becker, Bruguera, Kafelnikov, Rafter: 2
Gomez, Korda, Krajicek, Kuerten, Lendl, Moya, Muster, Stich: 1
Hewitt, Safin, Sampras, Kuerten: 2
Costa, Del Potro, Djokovic, Ferrero, Gaudio, Ivanisevic, Johansson, Roddick: 1
The current decade, which has thus far been solely dominated by Federer, Nadal, Djokovic, and Murray, has not been long underway and is subject to expansion at some point.
The conclusion is simple.
Bruguera, Kafelnikov, Rafter, Gomez, Korda, Krajicek, Moya, Muster and Stich are weak era mugs