Murray is absolutely correct. If the condition was faster and lower bounce Fed would have achieved even more. Fed is a better shot makers and would dictate the rally from the baseline. He gets more free point on his serve, the serve & fh comes in handy. His bh becomes a weapon, better striking zone and the slice is more effective.
OTOH, faster and lower bounce Nadal wouldn't have achieved as much. No way he has 10 slams by now.