Points...

"Closures don't have to be executed immediately or upon completion. Like any function, a closure has its own lifecycle, and capturing self weakly is the only way to prevent a retain cycle."

There's a difference between closure captures that retain references and retain cycles. If a closure captures self and is stored in that object, then you have a retain cycle and weak/unowned is needed to break it.

If you pass a callback closure to, say, datatask, and that closure captures self, then the closure retains the reference... and then frees it once the closure completes. There is no cycle.