That's also not true. The Dutch arose as an independent nation in the 1600s and share a lot of history with the low Germans. Even today groups like the Frisians have arguably more in common with Germans such as the Hanoverians than the Hanoverians have with Bavarians. They were different than other Germans but not significantly moreso than other difference within what we now call the German people.

A quick look at wikipedia also shows they came from all over Germany and parts of the Netherlands https://en.wikipedia.org/wiki/Pennsylvania_Dutch