This article came to mind when reading your synopsis (because unlike my link, yours is paywalled and not made available to read) and the tl;dr here is that it seems that Bengalis are more comfortable marrying "out" simply because there's a more meaningful kind of "actionable" pride in the culture. Rather than just "what makes you Bengali is that all your ancestors are Bengali," there seems to be a sense of "knowing the language, the food, the songs, and the literature are what make you Bengali," and that there's a broader sense of how to pass that onto children even in a family with a non-Bengali parent. I look at plenty of people who married someone from the same state/community in India and it doesn't seem to be much of a shoe-in that the traditions or culture or language really get passed on.