It does if you have blocking calls or you cannot yield to the event loop in time consuming code.

Are you asserting that single threaded execution of concurrent code is just as performant as multi-process execution?

Concurrency isn’t possible if there is only a single thread is my interpretation.

That is not true at all. Concurrency is a way of writing programs and is an orthogonal concern to how the program gets executed.

It certainly can be.