This is also about the App ID, actually mainly. So they keep K9 so that users can get a popup "export your settings, uninstall and install TB Android". As Android only allows updates with the same app ID and developer key.
They can probably even ease that out if they implement some API in K9 to let another app request data from it - Android has a system for letting apps send data to each other securely
IPC? I mean it can export and import settings, sending that data via the share portal and opening it for import (autodetecting the extension) is really possible.
Not that I know how to write a single line of Android app code.