The complement of as though expresses a non-actuality, a non-fact, just as the complement of as if does.
With the big holiday sale, it is as though an army of zombies was pushing at the shop door.
As a form of irrealis, it can be marked by a backshifted verb, as with was in the zombies example above.
When negated, BE not as though, it's often employed in contexts where someone is weighing the pros and cons of a situation and some convincing is required, that the "downside" is imaginary, not a real problem.
C'mon, man up! Just open the door and let them in. They're only bargain hunters. It's not as though they are an army of zombies.
I know it's something of an inconvenience, but if you can pick me up at 4pm sharp, I can be waiting right on the corner for you. It's not as though you would have to search all around for a parking spot and then come find me.
In your example, as the narrator is revealing Harry's thoughts, we see Harry in the act of considering the pros and cons of the better state for the Dursleys to be in, awake or asleep. Deciding in favor of "asleep" is not a mistake since...
It wasn't as though they were ever any help to him awake.