What Federer said IIRC, and what I think is true, is that top-players have less danger to suffer losses against lower-ranked players, typically big servers ... in those conditions.
It's easier for top-players to be so constant at reaching the late rounds.
It's not the only reason comparing to the 90s by the way : also the seeding system.
But I don't think that Fed did say that it helped him against the likes of Nadal, Djokovic and Murray, who have been his main opponents for long. He's not stupid.
I think overall Fed has mostly suffered of slower hardcourts against those, but I don't think he's suffered of slow grass overall because it's also helped him against big servers.
You just put yourself in a loop with those two statements. By that very explanation, he wouldn't have met Nadal, Djo and Murr as often as he did in all these years and they wouldn't have been his main opponents. Because faster courts would have defined more upsets a la US open 2012, wimbledon 2010 for pink and Wimbledon 2012 for Rafa.
So just so you know pinktards, the real victims are players below the top 4. Djokovic and Pink are least affected by slowness. Proven with stats.