DA was suppose to be in LA around the time JAckson was working out for the Lakers. Teams seem to gather for some players, not sure if they do it at other teams facilities, but it wouldn't surprise me if DA was at that work out and gave Jackson the promise.
I know every one is in aw with Fultz, but he's not locked down to be great. Jackson is more up DA and Stevens alley.
And now that Fultz is rumored to be asked to work out for the Lakers, I wouldn't be surprised if DA took Jackson at #1 or trade for the #3 pick.