Respectfully, that statement in no way states that the roommates did not see anything. If they had seen nothing, why would they assume that someone was passed out, as opposed to just being soundly asleep? I think they saw a roommate, lying on a floor, which would make someone assume that perhaps they were not just asleep. Alternately, I believe they may have attempted to get into the 2nd floor bedroom, and perhaps someone was lying against the door, so that they were unable to get it open enough to get in.
I still believe that they saw someone they knew was not just asleep or passed out. They may have described the person as possibly "passed out", but I believe that was because they could not accept what they knew they were looking at. JMO