YYZ does not close. It stops operating scheduled flights between 1am and 6am. That doesn't mean that if a plane is circling at 1:30am they'll let it crash, but I can imagine there are fines (as mentioned) which is why if the plane has gas at 5:30am they'll wait.
I imagine ATC and some baggage guys etc are always around in case needed, probably paid overtime?
Like other airports you also can't check in before about 4am. But unlike some airports you can indeed arrive and enter it at 2am, so it's not closed. Just not open for regular scheduled ops.
Back on topic... if AC gives three reasons and one is mechanical you are entitled to $$$ based on delay. It's unclear what costs if any were incurred due to the cancellation?