The appengine module was already in Furious ('ndb' is even marked as a default provider in furious.yaml) so I had assumed they weren't trying to avoid a reliance on appengine or ndb (which makes sense because the task queue stuff itself relies on appengine).  The core is still Context and Async and you can inject those with whatever persistence engine you'd like, my stuff just comes with a series of smart defaults to make using it easy.