Ok, I took a closer look at the analysis page linked to from the main article. There does seem to be a decent amount of Kotlin code written by Rabbit themselves, mostly UI for the settings screens, as well as some networking-related stuff (WebSockets, Wi-Fi, etc), hardware-related things such as the camera, OTA updates, etc. Basically, things that make sense to be handled closer to the operating system itself.

Interestingly, as far as I can tell, Compose is only being used for a "hardware test activity". The rest of the Kotlin side of things seems to use the traditional Android UI stack with activities, views and fragments. That makes three distinct UI frameworks in use on the Rabbit R1 - Flutter, Compose and traditional Android.