It's basically impossible for Google to open-source anything built in house, because it all relies heavily on Borg and various other proprietary tools and libra

It's basically impossible for Google to open-source anything built in house, because it all relies heavily on Borg and various other proprietary tools and libraries that don't exist outside the Googleplex. Pebble, on the other hand, was built fully outside Google and only ended up there via a circuitous route (the Fitbit acquisition), so this is not a concern.