They may be compelled to release any driver code associated, however firmware is not covered by relation to kernel. Linux runs on mostly proprietary firmware. The “linux-firmware” package in many distributions that contains hot plug firmware is mostly proprietary blobs.
That said I doubt they had much significant firmware work, it may just be logo and some tweaked configuration from their SoC vendor. They likely had to modify AOSP a bit more to allow their launcher unfettered access to the device in ways not modeled by standard AOSP, but that’s user space that isn’t GPL.