Pomorze is the actual Polish word for the region that in English is named Pomerania.
I don't know what the guy that responded to you meant saying that it's not in Polish. Sure, the region DOES have a different name in Kashubian...but we're talking here about Poland as a whole, not a minority group within it.
Maybe he thought you were mentioning the Kashubian name he had given (incorrectly according to three separate dictionaries).
Pomorze is the Polish name.
Pomerania is the English name.
According to the dictionary (my cousin would know, but I don't), in Kashubian, Pomorze is either Pòmòrskô or Pòmòrze (I assume they are just different forms of the same name, but again, I don't know the structure of Kashubian, though the Pòmòrze is the one more often given in dictionaries).