That's debatable, I'm American and would take an offer of a "steak dinner" to literally mean steak. Even presupposing you're correct that it's generally understood by Americans as an idiom, assuming that non-Americans understand American idioms is a form of ignorance, if not deliberate arrogance.

No one's saying "assume they are vegans," we're saying "don't assume they eat meat." Big difference.

I'm American and if someone offered me "a steak dinner" I would expect to be taken to a steakhouse, where steak was pretty much the only item on the menu.

I'm American, and if someone told me they're taking me out for a steak dinner, I'd immediately assume we were going to a steakhouse. It couldn't be any less of an idiom. If they took me out for pork chops, I would be completely and utterly confused.