If you look at the still, it looks like interference. But if you look at her arm swing leading up to the finish, Molly was taking longer and longer swings as they approached the line. At normal speed it looks like her arm was swinging back and got hung up on Chepkirui's body. Since it was on the way back when it hit Chepkirui, it just kind of hung there for a second. I don't think it was intentional and deserving of a DQ, but Chepkirui was closing fast and may have won without the arm block. But that's the price of running right on someone's shoulder.