Daniel made it clear that he saw Zo have a confrontation with another person at the party. Not only did he witness it, he intervened to break it up. On the other hand, Justin portrays a party that had no major issues outside of some bickering between couples. Zo arrived at the party with Justin; Daniel came separately. When you go to a party outside of your area, you tend to stick with the people you know, no matter how outgoing you are as a person. If Zo had gotten into a confrontation that night, Justin should have been/would have been aware of it. Yet, separately, each of the three Gardner friends interviewed claim to be aware of racism in that area and even speculate about a possible scenario where Zo gets jumped. Think about this...
One of the things unsolved mysteries actually left out is that they had actually investigated Justin's alibi. They looked at phone records, cell tower pinpoints, footage from an ATM at a gas station, and witnesses at a strip club he attended with another passenger in the car they failed to mention. Kind of made me wonder why they kept that information out of the episode.