IANA expert, but that is a radically different way of defining that term from what I’ve ever understood, read, or heard anyone else say.
IME = “Ethnicity” is genes, “culture” is what you behave or learned or practice. So in the terminology I’ve familiar with, your exemplar kid would be 100% indelibly ethnically Polish, but could possibly be culturally English if he really assimilated far more than most and affirmatively rejected the culture of his parents.