Federer himself actually admitted that it helped him.
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.