Not to diminish what this project has done, but they modified existing firmware to make it communicate with a different server. They've also implemented a server for the thermostat API.

It's pretty neat but, at this point, it's just a hacked firmware that talks to a different proprietary server.

Edit: It's not even a modification to the firmware binaries. They're just injecting /etc/hosts entries into the firmware[0]. If the Nest device just uses DNS to resolve these names then you wouldn't even need to modify the firmware-- just point it at a DNS server that's authoritative for the necessary names.

[0] https://github.com/codykociemba/NoLongerEvil-Thermostat/issu...