"Durkon will contact us" isn't actually explaining "the ship just left". I think a better explanation is: (starting from the previous strip)
Nice illusion. Now can we just get out of this stupid city already?
...Vaarsuvius?
It's no illusion, idiot. They're gone.
Wait, hold on: You're saying the boat ACTUALLY left us behind?!?
Keep your voice down! Durkon knows the plan. He'll contact us magically--and Vaarsuvius, too--as soon as he's able.
especially when you see it as Haley trying to keep Belkar calm (and thus, his voice down) by reassuring him that Durkon will contact them in the morning.