Yeah, sure if they had actually shut down the system. But all they did was discontinue outbound trains from one stop, O'Hare.
Inbound trains were still operating 24/7 and trains from the next station on the line (Rosemont) were still operating both directions 24/7.

There was no reason that the "clean" trains operating from Rosemont couldn't have pulled into O'Hare to pick up passengers. Certainly, given the minimal service during those hours, it wasn't an issue of track congestion. And they operated shuttle buses from O'Hare to Rosemont overnight, so it didn't really save resources.