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Doesn't look very native to me, you have to convert them. Could one jailbreak it and simply install the Kindle app?

There's no need to jailbreak them, they're open Android systems. You have full Google Playstore access and can install anything you want.

You can enable Google Play on Onyx Boox. Or just sideload the APK. Or install the Amazon Fire Store. It's just another Android device.