Nest Protect uses Thread (similar to ZigBee IP) which is not compatible with ST. However they do have an API and a DTH has been written to support it. I do believe that some of the Nest Protect features (such as motion detection) are blocked from the API.
It does also appear that you can use Halo with other wired smoke detectors unlike Nest (i.e. one goes off they all do) but if you want ST integration today I would go with Nest