[int_19h]

Look into continuation-passing style. Semantically, async/await is much like syntactic sugar for CPS (or rather futures, but at the most basic level they can be thought of as single-shot continuations).

But ultimately, to make use of async, you need async primitives - something that lets you say "do this in the background somehow, and let me know once you're done". Any async/await call should ultimately end at one of those primitives, and it's at that point that another call might get interleaved. If you don't actually do I/O or anything else that can do a non-blocking wait, you're not getting anything useful from async.

[ghusbands]

It's only semantically CPS in as much as all code is semantically CPS. Thinking about the parallels with CPS does nothing to help, here. Async/await is just like threaded code with the blocking/concurrent calls specially marked.

[int_19h]

Except it's not like threaded code, because there aren't necessarily multiple threads. And with a single thread event loop, you don't need locking and other synchronization mechanisms at all, further reinforcing the point.

Async/await is literally all about explicit continuations. It's not about concurrency or parallelism per se, although it can be used in that context.