I struggle with the idea that Dak played poorly last night, but I'm not sure this is much of an argument. Over a larger sample size I see merit to EPA/play. For a game like this one, and especially one where your QB only threw about 20 times it's pretty irrelevant.
Dak throughout his career has had some pretty bad games where he has racked up stats, and some of his most impressive performances have come when those stats were pretty ordinary.
From last night I saw a guy who could have had a better night statistically had it not been for a few drops, but there are a few of those where you could see that he just needed to drive the ball a little more into a receiver or just have slightly better placement. Had the Cowboys needed more from Dak last night he probably could have came away with stronger numbers, but he really didn't have to, and when you factor in game situations, weather conditions, etc I think Dak did just fine. I almost prefer this form of Dak more than the one who tries to throw for 400+ yards. When he can be in a less is more type of situation where the Cowboys can run the ball more than they throw it he is typically at his best.