The result is that Siri on your device has access to on-device information. Not Apple.While the implication above is that Siri is doing only local, on-device processing, the language doesn't say that. Even if all the processing is local (Apple outsourcing its AI load to users' devices?), the result is Apple gets the results to, among others, target Apple News stories at individuals. Whether that news is "cats vs. dogs" or "right vs. left" focus, it is personal.
Per the transcribed quote above, Apple says it is doing exactly that kind of harvesting, and from more than emails.
And, Apple absolutely is using on-device AI processing instead of processing in the cloud. But that doesn't mean that Apple is using your device to do other people's processing.