All knowledge of the outside world is taboo and books are confiscated. Since more than 100 years have passed the now alive population has no recollection of the world before the Wall. The present knowledge of the outsider and the history before, since there's clear evidence of control of information, is a clear constructed "truth". heck, that's the main point of the story, everyone's a lier, and some rather die than tell the truth because it's horrible! . For all we know there can be more survivors out there.