When Americans say they have roommates in SF do you mean you have to literally share the same room as someone you aren't in a relationship with?

While that can be the case, it's usually one person per room with shared common areas like kitchen and living room.

SF is so crazily expensive right now that it wouldn't surprise me to see people sharing a single room (with bunkbeds or something) just to save money.

I'm so happy I don't live there.

In my experience it just mean they're sharing the property, not living in the same room.

Ah. We say housemates to mean that, and roommates to mean if you are actually sharing a room (which wouldn't be likely).

Yeah - people don't tend to say "house" when they mean "apartment" here, so a housemate would imply that you are living in a house.