As a specialist in German Studies, I can confirm, it is Low German. Plattdeutsch refers to the dialect and Low German is the group of dialects, where it belongs, because of several attributes. Because Plattdeutsch is nearly the only representative of the Niederdeutsch, it is a used like a synonym. The name Low German (Niederdeutsch) comes from the geography of the places where it is used (in the lower lands). It has no negative meaning.
sapere aude!
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please vote my PNL:
https://www.behindthename.com/pnl/68992