You're right. In American English, the term "roommate" only means that two unrelated people share the same address, the same residence, not the same room.
Here are some other related words that you would hear in America:
bunkmate: a person who shares the same sleeping quarters (oddly, people who share the same room still more often call each other roommate rather than bunkmate)(bunkmate tends to refer to either temporary sleeping quarters or the military)
housemate: a person who shares the same house (used for referring to living in houses, not apartments or flats)
wallmate: a person who shares the same residence whose room is directly adjacent. (often used to describe dormitory living where two adjacent rooms share a single bathroom between)
flatmate: British. Americans don't tend to use this term since Americans don't tend to call apartments "flats." Americans don't say "apartmentmate."