That was disappointing when this thing also has the specs to be a decent gaming laptop if the GPU was usable in Windows. Not to say there aren't some great games available on Linux, but they're still the minority.
I suspect those of us outside of System76 can only speculate, but it definitely has to do with the firmware. My best guess is the firmware must be initializing the GPU enough to get it up for Linux but not Windows, or dual booting requires a boot path that does not currently support initializing the GPU. I believe, from my limited experience hacking with laptops, that rather than use the traditional mechanisms for initializing PCI cards, video cards on laptops, particularly hybrid graphics ones, need special initialization on the laptop firmware itself.
Something like that, yes. Coreboot lets the relevant driver in the Linux kernel do most of the hardware initialization. The firmware does comparatively little, just enough to make the driver recognize the hardware and configure it so it functions. The Windows driver for that hardware probably needs more handholding from the firmware.
It's refreshing to see the tables turned for once. GNU/Linux first, Windows secondary :)