I might be reading too much into it, but I took what he saw as a subjective vision, with Present!Pike seeing how Future!Pike remembers it... Mendez said Pike dragged all those cadets to safety, while in the vision he saw the cadets were able to escape all by themselves, and the glass wall was erected as he was trying and failing to save the last one inside. As I was watching it, I felt as if Future!Pike was beating himself up over his failure to save everyone, almost as if he could only see the one single cadet he couldn't save.Plus, in the vision Pike didn’t save all of the cadets, while in the Menagerie it was implied all were rescued, seems like Pike perhaps accepts his own fate to save all of the cadets.