‘Alt Hoch Deutsch’ (Old High German) sounds a bit like the Plattdeutsch my older Mennonite relatives spoke. But they left West Prussia for the Ukraine in 1802.
Plattdeutsch is a Low German language, and yes, it is an East Low German language with roots in far northeastern Germany and Prussia across the border into what is now Poland. It is close to Pomeranian, a dying East Low German language formerly spoken in that area that died out with the ethnic cleansing of the Germans there after WW2.
It is not intelligible with Standard German or really with any other German language, including other Low German languages. Low German is a completely different language from Standard German. German speakers cannot understand it at all.
Dutch speakers can actually understand Low German languages better than Germans can. That is because in some ways they are quite close to Dutch even though one is Old Franconian and the other is Old German. But there are also German “dialects” that are straight up from Franconian also, especially those spoken in northwest Germany near the borders of the Netherlands, Belgium and Luxembourg. There are dialects (or really languages in that area that are quite difficult to characterize as either:
I am thinking specially of the languages spoken where German, the Netherlands and Belgium all come together around Kerkrade, Aachen and Stolberg.