They've open sourced what they can, but these third party bits had to be stripped out: - All of the system fonts - The Bluetooth stack, except for a stub that will function in an emulator - The STM peripheral library - The voice codec - ARM CMSIS - For the Pebble 2 HR, the heart rate monitor driver
It's worth noting that CMSIS itself is open source but some of the drivers for this hardware probably were not.