That doesn't make someone German or mean they 'have German in them'.
I suspect most people would be a bit surprised if they did a DNA test.
German and Germanic mean two different things. When I read the OP's title, I was assuming he meant do the majority of Poles descend from the Germanic tribes.
I know but if Eastern Germans can call themselves Slavic, that means Western Poles can call themselves German.
Eastern Germans call themselves Slavic? Since when?
Sure, some eastern Germans have Slavic ancestry and some western Poles have Germanic/Celtic ancestry.
But that doesn't change the fact that eastern Germans are still Germanic linguistically and western Poles are Slavic linguistically.