Perhaps to help others -
https://www.reddit.com/r/Onyx_Boox/s/tw3S1L3QOE has a view of the inside of the Poke 2, which I suspect is quite similar. That shows the battery is behind the top of the screen, and confirms my suspicion that it’s a swollen battery, pressing against the screen from the inside.
I wonder where I get a replacement battery from?