What do you mean "And after that"? You said he was kept from facing ANY starters.
Each QB's preseason performance and position on the "depth chart" after training camp determined how soon and how much action each saw against the opposition's starting unit; Peterman from most accounts, was the most consistent QB during training camp: He got the first preseason start, McCarron was next, and Josh Allen---who the coaches were bringing along slowly---was last to start.
Peterman had a really good showing in his first start; McCarron put up pedestrian numbers before getting hurt in his first start and Josh Allen was granted the chance to move up the depth chart, in relief of McCarron ahead of Peterman; Josh Allen started the very next game, had a bad showing, and Peterman came in to relieve him in that game; McCarron recovered from his injury in time to play the final preseason game, and was given the whole game to make up for loss time due to the injury and to reestablish himself in the competition after his poor performance.
McDermott played each QB from a logical standpoint, based on what each showed.