If the Heat honestly believed there were players better than Winslow at 16th, and 23; then they would take it.
Riley is shrewd. He rarely anticipates or makes egregious mistakes without thinking any deal through. If Pat Riley was 100% positive, that there could be an impact player still left on the board, then it could happen.
I like Winslow a lot, and I would certainly do that trade any time of the day. If Ainge was so convinced he was worth the 4th pick, maybe it means something, or just a carefully planned out smokescreen.