Could simply be that, even if Blomkamp intends to overwrite A3 and A:R, Fox doesn't want anything in this book (despite it taking place post-A:R and in the original continuity) to contradict anything in regards to the style of Blomkamp's film, the abilities of his Aliens, etc. It'd be bad marketing to put out a novel, even if it's story has no bearing on the upcoming film, that conflicts with it thematically.