When I read that section, it occurred to me that "weak purity" is like monads and "strong purity" is purity as we know it in the FP world. Then drawTriangle() become an action in some monad (I imagined a State one) and renderScene() is just a pure function using some monadic computations (doing something like evalState (drawTriangle ...)).

I don't know how to explain that without analogies to Haskell (which, I presume, author deliberately avoids), though.