This is BS. Siri operates entirely in the cloud; the A4 CPU is not a bottleneck to its performance. In fact, it is well-documented that Siri started life as an app (also called Siri) written by another company, and that app ran perfectly fine on the 3GS, so we KNOW the hardware is not a constraint here.
Apple bought the Siri company, integrated it into iOS, delisted the original app (which was free!) from the App Store AND blocked the original app from talking to the Siri servers after the 4S came out (to prevent people who already had the app on their iPhone 3GS and iPhone 4 from using it anymore), and the rest is history.
-- Nathan