Activity-Centered Design & Scenarios

In your article Human-Centered Design Considered Harmful you say that "activities are not the same as tasks," but are scenarios the same as tasks? In my line of work (web application design & development) our personas are 80% scenario descriptions (one likely scenario per activity) — which we then break down into user needs, tasks and system features. From these we then build task-flow diagrams to describe an "activity" (e.g. registration), followed by prototyping. Your article is making me rethink this process, but I am keen to know where scenarios fit into the picture!