Holstein makes sense, actually. There's (according to infocard) high production of Industrial Materials for Rheinland and export to other houses, which means there's a huge supply coming from there.
Now, Iridum has a high number of industrial applications, mainly for high-temperature environments and fusion reactors (industry) and its corrosion resistance, thus its quite reasonable to see that Holstein imports it, considering that they create a big supply of Industrial Materials for Rheinland and other houses.
There should also be an unlawful sellpoint for it (Valetta?) to encourage unlawful cargo ships supplying unlawfuls with it, too and cause conflict between Casts and Corsairs in Delta.
