To be fair, the Sixers had the #3 pick, not the #2. Even if they assumed that the Cs weren't taking Fultz (not a sure thing), the Lakers were clearly also in the market for a pg and there was no guarantee they would take Ball over Fultz.
To be sure the Sixers got their man, they were willing to give up one of their surplus of plus assets over the next two seasons (while also protecting it in case they got luck with the #1).
There are plenty of Cs fans who think the Cs should have received more, but because Danny was never going to take Fultz anyway, he was happy to get something really solid in return, along with additional cap space.
Edit - I guess I took a little too long concocting my response. TPs to SL and gouki for beating me to the punch.