They use the data that drives the grades. For example, they can watch every snap that was a loss for Trevor Penning to see how best to attack him, which would require teams to have some faith that PFF's staff is able to identify a win or loss.
To me, it's funny that one of the biggest complaints of PFF is that they don't know a player's exact assignment on a play. News flash, neither do NFL coaches watching opponents' game film. Both groups are making educated guesses, but they can still see when a DL gets driven back, or when an OT gets completely beat on a passing down, even without knowing for sure precisely what a player was expected to do.
I wish I could find it, but there was a video where multiple coaches were asked to identify the pass coverage from the same game film, and they were all coming up with different answers. Even if they differ on what they think the coverage is, they can still figure out whether a CB has the speed to recover on a crossing route, or the physical skills to jam a receiver at the line.