There's some truth to it but I can't say if it is unique to Denmark. There's some phrasings and words used in Jutland that's completely foreign for people in Zealand. We also have our fringe dialects that are closer to being an entirely different language.

But what is true is that Danish puts way less emphasis on consonants and endings of words are often "swallowed" so to say, so for Norwegians and Swedes it might seem that we have a speech impediment at best. A joke in Scandinavia is that the easiest way to learn Danish is to learn Swedish and put a potato in your mouth while you speak.