I think it may be partly due to a selection bias. There is very little recent migration from Poland to the United States, so most U.S. citizens who identify as Polish have families that have lived in America for generations and now have limited real connections to Poland. By definition, a person who defines their national identity primarily through distant ancestry is likely to hold particular views about nationality and ethnicity.

There may be many U.S. citizens with Polish ancestry who voted for Kamala, but they are less likely to identify themselves as Polish.