That term actually makes the subject much more understandable.

It doesn't. "Task" has a notion of side-effect and no notion of yielding a value. Except a task can yield a value and have no side-effect.

and also the aptly named "TaskCompletionSource", which would be the promise equivalent (using Facebook's terminology)

That is, their terminology for this project. In facebook's Bolts framework for iOS they use Task and TaskCompletionSource https://github.com/BoltsFramework/Bolts-iOS#tasks