It is hard to say if the Sixers made a mistake or not. I agree that it was a good play to go hard to trade for Kawhi to help get LeBron but I am not sure LeBron was ever really in play for them. It feels like it was LAL all along.
If LeBron was never going to come to the Sixers, then it changes the calculus on Kawhi quite a bit. If LeBron and Kawhi were there, it is a good risk that they both stay. If they trade for just Kawhi, they give up some of their young core, probably still don't beat the Celtics, and then it seems likely that Kawhi goes to LAL or somewhere which would be an even bigger mistake for Philly.
I think what Philly did is give a lukewarm offer to SAS for Kawhi figuring they would up it if they got LeBron. They didn't get LeBron so now Kawhi is probably a one year rental for them. I would still give up Fultz but probably not Simmons or Embiid. Kawhi, Simmons, + Embiid along with whatever FAs they can pick up could make a run at the finals but are probably not better than the Celtics on paper. I would understand totally if Philly decided to back off on trading for Kawhi and just sign the best FAs that they can this year.