1) From another app, which obviously this author didn't do 2) Tapping the "+" icon on the top corner with a date in the calendar below highlighted. The New Event screen comes up with the highlighted date automatically filled in the Starts and Ends fields. This seems the most likely method if they were manually entering an appointment.

The point is, there is not enough detail to suggest that Apple listens to phone calls when there are other more plausible methods (and certainly more testable) that Siri could have used to obtain the information.