See, it turns out that the Rabbit R1 seems to run Android under the hood and the entire interface users interact with is powered by a single Android app. A tipster shared the Rabbit R1’s launcher APK with us, and with a bit of tinkering, we managed to install it on an Android phone, specifically a Pixel 6a.
Edit: Someone also got doom and Minecraft running on this thing
They wrote "lower level firmware modifications", AOSP runs on Linux kernel, and firmware modifications usually mean they modified the Linux kernel. This device seems like a regular Android phone, and afaik this rules apply to all Android phones, that's why Android rom cooking can exist.
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.
This was a marketing post, not a technical one. Unless we see any git branches or ROM teardown we won't know what they were doing. I highly doubt that they did any kernel patches though.