I agree with this. If a person has never lived in Poland then they can't know what life in Poland is like.
True, but that doesn't mean that you can change your ethnicity or race according to your country of birth, and it also doesn't mean that you can't adopt Polish culture within the family you grew up in. You are confusing nationality with ethnicity and culture.
In my case, I grew in in a Polish-speaking immigrant family, both my parents and even my stepfather were all from Poland. Our household would have acted the same way even if we lived in Poland - the only thing which would have changed is the social and cultural differences which exist in Poland, i.e. "the way things are done" in Poland.
However, even within our family there are diifferences. Of all the members of my family who were born here, I'm the only one of us who has a qualification in Polish and can read/write/speak fluent Polish.
My sister has none of the above, has virtually no understanding of the language whatsoever, hates the Poles and anything Polish, and appears as English as someone who can trace their family back to the Domesday Book. The only time she ever mentions her Polish origin is when she gets turned down for promotion and tries to play the "race card" (unsurprisingly, this doesn't work :D ).
But it doesn't mean that she has stopped being ethnically Polish
. The only reason that people like her can even get away with this, is because Poles don't look significantly different to other Europeans, and can easily assimilate into English society if they want to.
In this instance, though, I agree - she's really about as "Polish" as those 6th generation Americans who can't even pronounce thir names properly.
I suspect there would be less of this is we looked significantly different or non-white. Compare Polish assimilation to Pakistani assimilation, for example - even if they stopped being Muslim, and spoke in Received Pronunciation, people would still perceive them as "Pakis", not "English". If this was the case, not only would it be harder to hide our Polish origins, but we would also have more reason for protecting our culture from outside influences, like they do. People who are perceived as different or under threat, tend to seek out their own.
They can be proud of their Polish ancestry but they can't speak in the name of Poles.
They can't speak for residents of Poland
, but they can easily speak for people with a shared culture and language, if they adopted those aspects from their parents. It really isn't quite as straight forward as some people think - a lot of this is about the individual's perception of themselves, anyway.