I can answer this question finally. I asked Steffi from Fountainfeder. She said the German Diamine inks were commissioned by the German wholesaler/distributor SC Lucht.
They are also the main distributor for Sailor pens in Germany (and if I am reading the website correctly, also for Austria and Denmark (?)).
This makes the whole “exclusivity“ thing more understandable. It means that it’s not Diamine who decided to make inks for the German market, which was odd, but the inks were commissioned by a stationery distributor in Germany. That means they are exclusive to the company who commissioned them. Any German retailer who they supply can of course ship the inks anywhere they like.
@dragorsi, you asked on the latest FPC podcast episode about creating your own exclusive ink. The answer is you just have to get together with your buddies and commission an ink. Diamine offer the service, but I believe R&K do as well.
According to Nick Stewart though, it is not cheap to get your own ink made. SBRE Brown has done it too. So it is possible, but you need to get a large enough audience first.