Even if fully awake he may have had the sensation of having nodded off for a bit...
They were doing the approach briefing, so this is not likely. He knew that his attention was elsewhere for some time, and was completely startled by the unexpected change in the display when looking back. As several posts earlier noted, with the benefit of hindsight it is easy to say what should have (not) been done, but I really wonder how many would have fallen for the trap under similar circumstances.