My thoughts / speculation on the “uber” ride home, from most to least likely:
1. The girls took a Lyft (used ’uber’ as a general term, even to the sister. Uber has confirmed no official ride via their service)
2. The girls planned to take a ride-share, but instead saw a friend/acquaintance at the Grub truck and rode with them. Quite easy in fact to misremember ‘we took an uber’ when intoxicated when in fact they planned to Uber but instead rode with ‘Jane from class’ / ‘Sam from Chi Omega’
3. The girls rode with an ‘off-book’ Uber/Lyft driver. I’ve gone though their public Venmo accts - which both used at least on a weekly basis and no evidence of this. Could be they used cash app or Zelle for this, but I’d guess less likely
4. The girls took a ride from someone they didn’t know, as part of a campus service (e.g., when I was in both college and grad school at different institutions, both had ‘no questions asked’ safe ride services from students who would volunteer to drive you/your car home to avoid drunk driving). I give this one as a possibility just to show the range of plausible. I think this is unlikely.
IMO, the killer was not the driver.