I was wondering why he was on deck also. On a submarine, he would have been called to the control room or to the bridge at the first sign of a close approach in the open ocean. I agree with you though, his career is in jeopardy - and probably (deservedly) ended.
in this case “on deck” means he was on the bridge, this is where a surface ship is controlled from. Looking at the damaged bridge wing, my guess is he was there at the time of collision.
I suppose the sound of the collision horn woke him up. Both ships were probably doing more than twenty knots on their planned course. Merchant ships have very few people on the bridge normally, but even the most basic navigational radar should have sounded a collision warning. Reliance on aids to navigation is no substitute for vigilance.
The situation on the Fitzgerald is a little murkier. This should never have happened. Warship have the personnel, training and procedures to make this kind of incident all but impossible. Someone was not following procedures. There was almost certainly dereliction, and for this the Captain (may he recover soon and completely) is responsible. The sailors’ lives depend on him.
Don't know what happened here, but it sounds like the captain may have been on the bridge wing where the collision occurred.
Looking at the ship’s damage he could have been in his at sea cabin, located close to the bridge. Night steaming orders for the bridge crew will
often include a standing order to wake up the Captain for any vessel traffic within a specific distance.