I had no idea that is possible??? Is that special for Germany?

I mean, the regulation seems to be, but there's no fancy tech going on. I'm not an electrician but I think I can explain, as I have recently tried to understand myself. To understand why it's possible it's best to understand why the limit is at 800 W precisely.

So German wall outlets usually have a 16 A fuse and the wiring in the walls is dimensioned to accommodate slightly higher current (I think they are 2.5mm² gauge allowing up to 20 A but don't quote me on that particular part) for safety reasons. I suppose it would be the same or very similar in Denmark, or maybe most of Europe that uses 230V/50Hz AC.

Now, normally, if you have dangerously powerful load that would melt your wires, let's say 5 kW, and you plug it in to an outlet the fuse will just pop and you're safe. If however you have a 2 kW PV system connected to a wall outlet nearby, it would theoretically be possible that your 5 kW load draws 13 A (3 kW) from the mains through the fuse and another 8.7 A (2 kW) from the PV system over the same wire in the wall that is only rated at 20 A but now carries 21.7 A. And the fuse would never pop at 13 A, making it a huge fire hazard. 800 W is basically just what will always comfortably fit into the safety margin of the wiring in German houses. All systems above 800 W need to be hardwired by professionals "behind" the fuse box so that every Amp from your PV goes through a typical 16 A fuse.

still such an 800W system is dirt cheap by comparison

Absolutely. I guess the low threshold for installation allows some kind of mass market economy of scale whereas systems like yours are homeowners' luxury goods.