I see the following paragraph in the monthly update:
Wikipedia's Android application has been updated with the ability to access ZIM files, but we are not promoting this feature at the current time. If you have ZIM files on your device, which you can get from Kiwix, you will see the feature in your app.
But I suspect the validity of this. AFAIK, the ability to access[1] ZIM files is only available as an opt-in feature in the alpha version of the app. Further, I don't think the app automatically enables the feature when ZIM files are present in it but it is provided only as an opt-in feature (but automatically enabling it seems to be a nice idea to me).
I'm not sure about this so Cc: mobile-android-wikipedia@wikimedia.org
-- Kaartic