You started a sensitive topic, so please keep in mind that all these replies are not here because we want to make you feel bad, but just because most of us have some bad experiences here.
I think you are a responsible dog owner, which is also shown here, when you try to inform yourself and asked us for an advice.
I think the next time you are going to be in a situation like this (when someone is trying to tell you something and even more so if he is yelling at you, you should predict that something isn't right… I don't know how good your German is, but maybe he tried to tell you if you could please keep your dog on a leash and you didn't hear or understand him, so he started to yell because he was probably afraid.
You said his dogs were reactive so I guess he probably was in a similar situation before and because his dogs bark and behave reactive, other dogs take this as provocation and bad things can happen then.
Me for example, I talk very calm and on a silent side, but when I am afraid for my girl, I would yell too.
You also said your dog is afraid of bigger dogs… well, my is also and I always keep her on a leash. And I can’t count anymore how many times I asked politely if people who were approaching us could leash their dogs. Some did leash the dog, some didn’t (and in that case, I raised my voice and repeated). Someone even told me that my behavior is idiotic (exact words) and that I shouldn’t have a dog. Most often people just roll their eyes, grab the dog for a collar for a few seconds and make few steps forward and when passing us they told me that their dog won’t do anything and he just wants to play.
Well, my girl gained the fear for bigger dogs just because too many of “they just want to play dogs” “played” with her and guess what; it was always “the first time” their dog hurt someone. And even then it was my fault as I have the breed which other dogs usually don’t like, so I was told.
So, although you know the laws now, please keep in mind that it is polite to leash your dog when you are passing by other dogs. It takes just a second for you to leash the dog if she is at your feet anyway.
wish you happy walks and waggy tails!