It's an ET in the sense it goes on what happens during the play, and just like all ETs it's based on how one interprets the value of a player's activity/actions.
Just because a player receives a less than stellar grade doesn't mean they had an average game. As they've noted, "The greatness of Rodgers’ performance last night was in the intangibles. Recognizing the blitz, drawing the defense offsides, catching the Chiefs in bad situations and exploiting those scenarios with simple passes to open receivers. But you cannot — and we do not try to — quantify intangibles, or what comes pre-snap. Our system grades what can be graded — the execution of the play post-snap — and in that regard Rodgers did not stand out in the same way that his statistics did.".