I think what he meant with the lobster case is that a BT is used to having eaten it his whole life. This concept has a basis in halachah by Pesach where there are harchakas on account of us being used to eating it our whole lives.
I assume he was talking about a “fresh off the boat” BT. If someone would make this presumption about a person who has been a BT for many years it would certainly be demeaning.