Obviously, And those who had that opinion said things like that. Their opinion was probably based on Federer's utter dominance of the tour for 3 years which made Sampras' stats look second-rate.
In general though, when commentators say "we might be looking at the greatest player bla bla", they don't mean that if that player retired tomorrow he would be the greatest, but that after his career he might be in that position.
In understand that because it is common sense. As we can clearly see, someone else lacks the ability to use it.