His point is that when siri asks you if you want to search the web, she gets into a mode where she fails to understand things she usually would.
Siri is designed to be more human than most interfaces, to be less modal, and require minimal prior knowledge of how she works and what you "need" to say. However in these circumstances she becomes very modal, and more restrictive.
A simple "start over" button would IMO would make things flow better, and help satisfy those design goals.
Pete
great posts! I never knew I can tell her "Start Over" to avoid this