That’s because Siri doesn’t run on-device - phones like the iPhone 6 can’t run that level of analysis. They “collect transcripts” insofar as they need to process your request.
Nonetheless, Siri is trivial to disable altogether.
Yeah if you asked the average person on the sheet (e.g. you) if they thought Siri was 100% private, they'd say yes because Apple has mislead them and said it is. That's the point. Apple says everything is private but then secretly collects data via their privacy policies.
Nonetheless, Siri is trivial to disable altogether.