Well statistically it's far more likely that a person who is Asian will practice an Asian system of religion like Buddhism (or Shinto, Hinduism, Daoism, etc.) in the US – best guess is upward of 96% chance. Just like there's an 84% chance that someone mono-racially Irish from Ireland is Catholic. Just like there's likely over a 90% chance that someone who states themselves to be a Jew (not by marriage or conversion) in the US likely carries the Ashkenazi Jewish gene (Yes, there is a genetic test to determine this ethnic variant, and also there are a slew of diseases that are proven to occur far more often in the ethnic Ashkenzim population). So really, it's not a stretch for a person to wonder if someone with Irish features might be Catholic; or to wonder why someone who has certain features that visibly reflect thousands of years of limited breeding among Eastern European (Ashkenazi) religious Jews, just might be Jewish; and it's also not a stretch for someone to hear about a family of Buddhists moving into their neighborhood and logically inferring that they're most likely Asian.
… What's problematic is someone looking at someone Sephardic (Middle Eastern or North African Jew) and refusing to accept that person as Jewish because they foolishly believe only White Jews are “real” Jews because they are – for now – the group of Jews most politically and socially visible in the Jewish diaspora. It's problematic when certain people insist that a president must be a Muslim because of his name, though he has embraced the Christian faith. It's a problem when someone states that the family of Buddhists who just moved into their neighborhood can't be “real” Buddhists because they aren't Asian.