As an Australian in the US, it seems that term is ambiguous.

When a college student talks about their roommate, yeah that's the freaky situation of being an adult and not having your own bedroom.

But on other contexts it just means what I would call a "housemate". You're not actually in the same room.

This is not true. Sometimes it refers to this. The vast, vast majority of the time, they are referring to someone who has their own room in the same apartment. Source: I had "roommates" in the way you refer to it in college, and after graduating, most people who I describe this to are actually shocked that this happened after freshman year.

Nah, that's pretty exclusively a college dorm thing, where it has a pretty direct relation to the price of the dorm.

(my uni had both twin and single rooms, and the twin was maybe 60% of the price)

Only in dormitories. Roommates in an apartment don't normally share rooms.

We don't. I'm an midwestern american college student and "roommate" means "someone that shares your living space" whether that be sharing a room to sleep or just sharing kitchen/bathroom/living room space. The only alternative I've heard is "suitemate" in suite-style dorms.

That's not true. Roommate almost always means two people sharing one apartment with their own bedrooms. Dorm-mate, on the other hand, specifically means two college students sharing a dorm room with two beds.