According to the article he was sitting in a chair in the skyway. Access to the skyway is determined by the buildings it connects, and those buildings can close skyway access to their building during non-business hours. But so long as the skyway is open people have free access to it. It is not a lounge area, it is not an employees-only area. If there is a bench or a chair in it then there is no reason why they can't sit there.
You are correct. IIRC, the skyway locks down at 1am.
(most of the places of business close much earlier, except the bars)
Or at least it used to when I lived in that complex many years ago.