IMO, the schedule had a lot to do with it- it gave the Sixers time to get it together against lesser competition. Then again, the win over Portland suggests that the team is significantly better than average. San Antonio will be a better test.
I put DiLeo at the top of the list, since the team is definitely playing a different game. Cheeks never played Theo, or relied on Marreese at the end of the game. Cheeks never ran a play for Thad Young. Cheeks never told the players to take open three's. The team is much, much better under DiLeo, they never shot over 50% from the field when Cheeks was coach.
Not just Iggy has improved, but nearly everyone with the exception of Willie Green.
Confidence comes from winning. They are getting to be very confident of their ability. When Portland made a run, it looked dicey for a while, then DiLeo made an adjustment and they were off to the races again.