I'm fairly sure I read somewhere / saw in a video that it's FreeRTOS on an STM32-series microcontroller.
It could perhaps be this video:
https://youtu.be/OZZ3zm3-xr8 by Charbax, featuring Dr. Janko.
At 6:27, Dr Janko starts talking a little about the cover display.
At 7:35, he mentions it runs on a STM-32 and also that the screen is 2" and 240x536 pixles.
At 19:46, he mentions the CoDi is running their API on top of emWin from Segger, which in turn runs on FreeRTOS.
At 20:11 he says "We'll open up the API so people can use it" (is in the context of the Linux developer community).