The yield keyword was also used in green-thread coroutine / cooperative multitasking systems simply meaning 'yield control back to the task scheduler'.

> Even weirder to say its a "C# coroutine" feature when there's no coroutine in the C# language.

Unity has a coroutine system based on iterators, maybe that's where the confusion is coming from:

https://docs.unity3d.com/Manual/Coroutines.html

...or simply because async/await, coroutines or any other multitasking system serves the same purpose for the user: to kick off background tasks.