Hello Lars,
importing from InDesign makes perfect sense for many publishers who want to convert their paper books already made with InDesign to a digital and interactive format. We know InDesign allows to export fixed-layout EPUB on their own, but moving to PubCoder allows to add interactivity, read aloud, multiple languages, and export also as native iOS/Android app, html widgets or XPUB.
We're a small team, so we need to carefully select what features to implement. InDesign import was added because a lot of users asked for it. At the moment, there's not such demand to import from the tools you mentioned. But maybe we will add support for that some times later, though it is not actually on our roadmap.