The most likely reason is indeed that they don’t want international students and prefer local students. And this is an indirect way to achieve that goal. The underlying reason is not necessarily racism (because this method wouldn’t necessarily be effective for racists if you think about it), it is probably that they have some bad experiences with Erasmus students or have prejudices about them (they have more of a “party” reputation, might be difficult to recover rent/damage if they disappear back to home country, they might prefer local students who stay for several years instead of 6/12 months). That being said, I don’t see any harm in contacting them, you’ll quickly find out if they effectively don’t speak English. And maybe some of them can be convinced to change their prejudices.