I believe he was targeted twice... one he was beaten by Metcalf it was an overthrow (he gets graded on that even though it wasn't complete) and he had a pass interference on another WR. So in those limited targets he didn't do well. As you say they have no way to measure him completely eliminating Metcalf for the rest of the game.
So yeah its flawed for sure. Here's what you need to know about why the ratings are good... because DW clearly was targeted and when he was graded through the roof.... which is why his grade was absurdly high.
