Couple of things. One Quarterbacks don't read receivers as much as they read where the receivers route is going to end up.
Escobar has leverage on the LB who is covering him and the nearest defender in the direction of the route is outside of Witten on top of the numbers. Witten appears to be running a seam route, so the that should drag the defender that is on Witten out of the space where Escobars route will end up.
If the defender that was on Witten sits ( I don't remember how much pull Witten had on that defender), the space between the end of the hash mark and the numbers is 15 yards. Even if Escobar was not open, with that much space Romo could have thrown Escobar open. But Escobar had leverage on his LB and with 15 yards of grass to throw into, it should have been an easy pitch and catch. They would have had a manageable 3rd down, the decision to throw that ball was sound.
By NFL standards Escobar was open.
Edit: Witten ran a corner route. I was mistaken because I heard Garrett and Romo talk about having a couple of receivers run some seams. Although it could have been a seam for Witten with the option to run a corner. The defender on Witten left just as the ball was thrown, but like I said, the space between the end of the the hash marks and the numbers is 15 yards. Ample space for a receiver that has leverage on his man.
