The old form of Neder-Hollands, more related to ‘Vlaams’ (Flemish) is ‘Diets’. It’s the base language for Flemish, Dutch, and German. Related dialects are especially heard in Belgian Limburg, Netherlands Limburg, and Nord-Rhein Westfalen (Kölsch). The Luxemburg language(Letzebuerg) is closely related.
Per my Merriam-Webster’s, Dutch as a word entered the English language in the 1400s as a term to loosely group the Germanic peoples and languages of Germany, Austria, Switzerland and the Low Countries. In the US, at least, we also have the Pennsylvannia Dutch, who, you guessed it, are from German and originally spricht Hoch Deutsch. So we have Austrians, Germans, Swiss, Flemish and Dutch. Why not Nederlanders or Hollanders? Maybe we couldn’t pronounce it. Just a guess.
Im not so sure the Pennsylvania Dutch at any time spoke Hochdeutsch. I could be mistaken here, but I was under the impression that Jacob Aman and his followers came from southwestern Germany and Switzerland. There are very heavy dialects spoken in these regions.
In any case, Hochdeutsch sure as hell isnt spoken by the Amish today.
Sorry that I left out the part about it being a dialect. IIRC most German currently in use in German is dialectic variations of High German. Personally I had a lot of trouble to understand the German spoken in southern Germany, and in Switzerland (* Schweitzer Teutsch*), fuggedaboutit! Sorry for the confusion.