I'm guessing the lack of APIs is due to the Assistant being in a preview state, and that we'll see the start of a slew of them on Oct 4, when things like Google Home are released (and are also powered by the Assistant, according to the assistant if you ask it what Google Home is).
Right now, you can ask the assistant to turn on/off lights, call a taxi, etc, and it just responds with "I can't do that yet."
The Google team also added the ability to not return the same response for any given request, unless the request falls into generic search. It's not necessary, but gives it that human kind of element, and keeps it interesting.
I didn't try any kind of logical reasoning since it seemed to be heavily reliant on Google search, which is still keyword based to a degree. One day this will probably change, but it's a hard problem to solve given an open domain.
I thought I would see more direct integration with APIs as opposed to defaulting to search. I don't think I saw any in-fact beyond what is offered through search. EG - book me a ride with uber just goes to the search results.
I'm wondering if this team is working together with the home product. If there is also a heavy reliance on search, I can't imagine the user experience will be that great.