Flashing the high beams or using your horn is allowed in two situations: to signal that you will overtake the car in front of you (or have the intention to do so) outside of a built-up area, or if you judge yourself or someone else to be in danger ($16 StVO).
There are some additional provisions about not blinding oncoming traffic, but that shouldn't be a concern on the autobahn. Signaling something that they are driving unsafe and put you in danger is justified by the law.
My experience with German road police it that they can be really obnoxious and rude. Me and a friend got stopped once in Germany, and we were picked out solely because of our Dutch license plate, nothing more. Guy demanded to search our car, demanded we open and empty all of our luggage. He was sure he'd find some marijuana since we're Dutch. Held us up for more than two hours, threatened to get a sniffing dog (which he hoped would make us give up what he suspected we had), and just being an enormous douche. He then just left us standing next to the highway on a very dangerous spot with all of our luggage a mess and on the street.
So yea, probably he just had a bad day. Or, he's one of the many many men that work for the German Police that are just enormous a-holes.
Flashing the high beams or using your horn is allowed in two situations: to signal that you will overtake the car in front of you (or have the intention to do so) outside of a built-up area, or if you judge yourself or someone else to be in danger ($16 StVO).
There are some additional provisions about not blinding oncoming traffic, but that shouldn't be a concern on the autobahn. Signaling something that they are driving unsafe and put you in danger is justified by the law.