Reading back I see nothing that suggests that RYR's flight crew flying ex Germany were actually domicile in Germany so please define what you mean by "based"?
Based means where you start and finish your duty. The majority of pilots are not floaters and spend most time in one base. It's not complicated really and by looking at someone's roster it should be obvious where they're based. when I recieved my base I got it written on paper. It's a FTL requirement. It may be far from where you are domicile, yet it defines where you should pay social taxes.
I agree with you, someone stepping through the door at STN, positioning at the behest of the company, flying out of HHN, positioning back to STN is not based in Germany. However, you are talking about a tiny fraction of pilots. Contractors based in Weeze, Hahn, Baden-Baden, Bremen and whatever more there is now is what you should be focusing on.