Drew may correct me on this, but I dont think he is necessarily talking about the BDUF approach from a traditional software development viewpoint. ie. not documenting every detail of the system from a developer-standpoint (eg. this function must be AJAX-driven, user details will consist of x and be stored in a mysql innodb database table etc), but more as complete-as-possible account of the user stories the website / application needs to satisfy.
I've managed development teams that have very successfully implemented agile development work within a larger organisation that follows waterfall within its general IT teams. The trick is two-fold. One: remove low-level system dependencies (do you really care if i use Apache 2.2 or IIS to host the website??), and two: internally tier the requirements to allow iterative progression and evaluation.