I think you're just supposed to fill in the circumstances with nameless horror because that's pretty much all the subject seems to have experienced. When reading I was thinking about the lack of specific detail and the limitation of the main character's viewpoint being our only insight into the story's world. I would say that the goal is to put the reader on the same level of awareness as the thing in the basement and allow our minds to fill in whatever is upstairs. That is what makes it most relatable to the real world; our experiences will conjure up things we abhor to fill the gap in detail about what's outside the basement.
I don't see it. We're told what's outside the basement: the "parents." But we're not given anything to indicate why they react the way they do.