Initially doing internal integrations, then releasing API access to trusted partners, then making APIs publicly available is how Google has done lots of things. So, I wouldn't be surprised if that's the route Google does with this.

Yeah, I suspect the idea is that "public APIs are forever," especially in hardware. They probably want to be able to collect some real user data, make a few mistakes and get a better idea of what role the product is actually going to fulfill before committing to something that they'll have to maintain indefinitely.

That's not what they said in the keynote. They were explicit about the fact that developers would be able to extend it. They used Uber as an example.

I wonder what the point of even announcing it at a dev conference was, save it for CES

If true.... seriously Google?

The key word there is initially. Echo didn't have an SDK when it was first released either neither did Google Now for a while. That's Google's MO for new services and APIs, limited initial release to iron out the bugs and not flood with low effort apps/services.