Sorry but that's completely wrong and a typical Northern misconception.

Luther based his Bible translation on the language of his Duke's bureaucracy called Meißner Kanzleideutsch, which itself is a compromise of Central Eastern and South Eastern dialects but certainly not Northern.

Otherwise it wouldn't be called High German (it's about altitude not quality)

Your Plattdeutsch is referred as "Low German" in linguistics. Please keep in mind that "platt" is also a term for dialect in a much wider part of Germany, for instance Rhöner Platt