Sure, a common use of bread, potatoes, cabbage/other vegetables, hearty meat dishes etc but the Polish kitchen is closer to Ukrainian/Russian in technique/ingredients.

Barszcz, pierogi, fermented everything, pickles, sour rye, and many dishes built around wheat/rye, mushrooms, dairy, and Eastern-style fillings are much more like Ukrainian/Belarusian/Russian food.

The biggest German influences are probably the sausages and the beer culture.

It's also true for Belarus, Baltics, and some parts of Ukraine. Generally, we can speak about North-Eastern European cuisine with potatoes, secale flour breads, and various pickled things. And that name will make a lot of everybody upset, cause everybody in those lands pretend they are "Central". Americans would not believe how many "geographical centers of Europe" are claimed there.

How reasonably can German cuisine be described as a single unified thing? My mother was from East Prussia and my father from Swabia and their "home" cuisines were pretty dissimilar -- if for no other reason than climate.

Due to Partitions of Poland a lot of of territory was under Prussian influence for over a century - that had to have some culinary effect (other than forced germanization).

Lots of common main ingredients like potatoes, beets, cabbage, and sausages. It could also have a different reason, like https://en.wikipedia.org/wiki/Former_eastern_territories_of_...

Yes it's similar, but certainly not more than Ukrainian/Russian/Belarusian food.