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)
cellmate: a person who shares the same prison cell
suitemate: a person with whom ones shares a suite, as in a college dorm; roommate; also, a person with whom ones shares an office suite
wallmate: a person who shares the same residence whose room is directly adjacent but not part of the same suite. (often used to describe dormitory living where two adjacent rooms don't 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 commonly say "apartmentmate."