IMO it's likely just police procedure to say, at this early stage, that they can't rule anyone in or out yet regarding the other roommates.
I do see, in MSM above, that right now LE does not consider the food truck guy to be a suspect or POI.
While I agree that anything is POSSIBLE, IMO it's highly unlikely that one or both of the surviving roommates had anything to do with this massacre.
I know SM often paints a false picture, but it does seem like all five of the females got along well. Generalizing, I know; but when girls fight, they tend more to shun one another, or make catty remarks, freeze the others out, and insult their looks.
IMO if the surviving roommates had any issues with the four who were killed, it's more likely they would have sought other accommodations and been done with it. Maybe give them dirty looks around campus or spread rumors about them.
The other two girls are slim and petite and this would take brute force.
I just don't see it, at all. I think they are deeply, deeply traumatized. To lose your three roommates and the boyfriend of one would be traumatic even if they'd died in a car accident. To know that they were murdered, murdered while you were present, murdered and you could've been next--I'd be catatonic.
Jmo