I think that in your sentence, "the task" is the task that calls withUnsafeContinuation. There is a second task that enters the equation, and it is the eventual task that is resumed by withUnsafeContinuation (as a suspension point).
This second task can start running before the closure has returned. For all we know, it may even start running on the same thread as the first task, before the closure starts. This uncertainty raises questions: should we assume that methods that call withUnsafeContinuation must support reentrancy ? To be explicit: if the second resumed task immediately calls the same method that is still inside withUnsafeContinuation, waiting for its continuation, then this method must support reentrancy. And what if we don't want to (support reentrancy)? And which expectations will break when users will be able to write their own executors?
Unsafe continuations are still not sufficiently documented and described. This is all very confusing, and a lot of people are writing buggy code thinking they are safe. One bug around unsafe continuations I found today in the Swift runtime: `withUnsafeContinuation` can break actor isolation · Issue #61485 · apple/swift · GitHub