Have you actually watched the play? 85 is open and his defender is stumbling while Dak is standing in the pocket staring down CD. 85 did a good job of breaking in front of the CB face that was playing inside leverage and had a couple yards separation by the time Dak threw the ball. That was the right read. A safer throw and would have led to points. It was just not the throw Dak wanted. Thus he never looked at anyone else but CD. He determined pre snap where he was throwing the ball regardless of anything else.
Of course I watched it. Did you? Here...
To begin with, there are several questions on that 3rd & 15 play.
1) What was CeeDee doing running a fade straight into double coverage with a DB trailing him on his heels? CeeDee would have been wide open had he ran a post on that play.
2) Why did Dak throw to the spot he did which was behind CeeDee? Dak doesn't throw this far behind a receiver unless he was expecting CeeDee to run a deep comeback or curl on that play.
3) Dak had pressure right in his face in less than 3 seconds after receiving the ball. Tyler Smith got pushed right into Dak's face from the snap of the ball and in less than 3 seconds of time. Since it came from Dak's left side, it would have been hard for he to see Noah Brown breaking late from the far left side of the field and towards the middle. Had Dak waited to see Noah clear in front of Dak's view, Dak would have been sacked thanks to Tyler Smith getting bullrushed back into Dak's face in less than 3 seconds after the snap of the ball.
4) Yes, you're right. Dak was predetermined to throw the ball to CeeDee, and it would have worked had CeeDee ran a deep comeback or deep curl which would have resulted in a 1st down. Either CeeDee ran the wrong route or Dak tried to force it to CeeDee.