Also your map proves that the southern Germany was NOT inhabited by Germanic tribes by this date. Can yopu show me which tribes were living there during 304 BC?
So who lived there back then?
We don't know. Most likely some unnamed celtic tribes we don't know about. That's the reason why it's populated but no state is there.
But why? What was different about this region, compared to the adjacent regions?
We don't know ANY state or tribe who lived in this region. The devs would have to made them up and add fictional tribes.