Yup, this. Siri does not have access to your database, nor can it index it. It's server side. That would be invasion of privacy to allow that. Imagine if Apple could remotely link in to the information on your machine and index it. Yeah, no. I love Apple but it's good they don't do that. It would be a huge welcome sign for the likes of the NSA and other users who wish to exploit such a vast door of fun....
So what happens when you say "hey siri, call mom on speakerphone" ? And does that work without a data connection? yeah ...