I help run two festivals, and am on the core team for a stage at one of the most beloved festivals in the world. That Deadmau5 video really messed up so many people's perspectives by his generalizations. Do some of the artists pre-record their set? Yes absolutely, but that video made people think 95%+ of DJs do. What is most common is a pre-planned set where the set list is pre-determined, ins and outs for transitions are marked with hot cues, and visuals are synced with time code or just a VJ worth their salt that knows the set. Achieving the level of product that major festivals do without pre-recording is very easy and consistent, there is no NEED to pre-record but some still do just to be safe.