Siri was developed by SRI International, basically as the commercialization of the CALO project [0]. It was initially developed as an app, and was even available in the store. The project was then spun-off into a company, and the developers were working on versions for other platforms. Apple then bought the company, making it exclusively an iOS product and integrating it into the OS.
The voice recognition technology was indeed based on Nuance technology [1]. Nuance was also a spin-off from SRI.
EDIT: This was all after ScanSoft purchased Nuance, which means it had both Nuance and Dragon technology at the time. However, due to Nuance's history with SRI, I would lean towards that being the more likely option, if they hadn't been integrated by that point.
(I worked at SRI during the Siri spin-off time period, and worked with one of the Siri developers on another project.)
It's not a fact : "The Bakers believe that some of their technology made its way into Siri."