I don’t think that’s exactly true. Only InkPad X Pro, InkPad EO and Color Note are on Android, but I think all other readers are Linux (non-android). On the other hand, some services only work with iOS or android (for example the Finnish e-library) so this may also be good to keep in mind (I had no idea pocketbook even have android versions until now).
If you haven’t updated your kindle to 5.18.1, you can also jailbreak it and install koreader which allows you to read epub natively on the kindle, see https://kindlemodding.org/jailbreaking/WinterBreak/