We don't really know the timeline yet. The affidavit says the car was seen speeding away from the residence at 4:20am and returned around 9am. BUT it also said one of the victims received a Doordash order at 4am and was on TikTok on her phone at 4:12am, so she couldn't have been asleep. Brian likely came by around 4:20, saw that the lights were all on, and took off. He came back later in the morning, everyone was asleep (which they would be, after being up until after 4am the night before) and did the killings. The roommates who were still alive hid in their rooms for a couple hours, at most, before coming out and calling 911.
The police reported things a certain way to not lead on about how much evidence they had. Their early reporting was likely not completely forthcoming. They correctly assumed he was still in the area and didn't want to endanger the remaining roommates. Saying the roommates slept through the attack and didn't witness anything kept them from becoming targets.