That is not true. Siri was available in the App Store from a third party for at least a year prior to its reintroduction in the 4S, and ran on a 3GS just fine. There isn't a technical reason for it not being available on earlier models.
Define "Just fine." The argument is that Apple decided not to release their version of Siri on the 3GS because a hardware difference meant that the voice recognition accuracy would have been poorer.
You can't really counter that claim with the simple observation that an earlier version of Siri was available for the 4 and 3GS; that's just talking past the point. A more appropriate counter would be a source saying that the voice recognition accuracy of that version of Siri was just fine. Or at least no worse than what ended up being released on the 4S.
You can't really counter that claim with the simple observation that an earlier version of Siri was available for the 4 and 3GS; that's just talking past the point. A more appropriate counter would be a source saying that the voice recognition accuracy of that version of Siri was just fine. Or at least no worse than what ended up being released on the 4S.