Supposedly some info is gained from cable boxes as well and not just Nielsen ratings or so I have heard. And DVR viewings help too.
You are right though that they couldn't have expected too much for where it aired. But maybe the ratings were too bad for even that slot? We can argue that other networks air encores, rerun timefillers, or infomercials, but Adult Swim is a late night network, so maybe a slot like that is more important to them. I mean, Adult Swim actually airs lesser shows and reruns during prime time because they can't compete and then ramp up with their big shows later on.