I don't have any skin in the game (I'm using a Pixel right now, have a ProductRED iPhone 7, and have the S8 on preorder), but the point is that we DO use these displays and there are other variables. So from that perspective, DisplayMate making the assertion that the S8 is the best -mobile- display based on their testing isn't wrong. If you take all those other factors out of the equation then yes, the iPhone is more color accurate and thus objectively the better display.
It's like saying a phone has the best camera because it takes the best still images, but isn't very good at video and dismissing another phone that is only a fraction worse on still images but far better on video.