I don't know what sort of proverbial (or literal) shape the dumplings took at this eatery, but I think a lot of people also forget that dumplings themselves are practically a culinary universal, with SO many different cultures having their own version of the concept of "dough pocket with food inside." They come in many sizes, shapes, fillings, and prep-methods (e.g., fried or boiled).

There are mandu and gyoza and shengjianbao (and more, just from East Asia alone), sure. But then there are dough pockets like samosas (South Asia), or ravioli and tortellini (Italy), or pierogi (Poland), or Maultaschen (Germany), or empadas/empanadas (the Spanish- and Portuguese-speaking worlds), etc.

Tl;dr: No one on this planet has a cultural-moral monopoly on the concept of dumplings.

In fairness, depending on how the dumplings were presented, perhaps the owners did commit a faux pas (again, I don't know; I haven't seen them)... but to assume that "dumplings" are unequivocally a sin of appropriation each time a non-East Asian person makes them is...... bizarre.