async let and TaskGroups are not parallelism, they're concurrency. They're usually parallel because the Swift concurrency runtime allows them to be, but there's no guarantee. If the runtime thread pool is heavily loaded and only one core is available, they will only be concurrent, not parallel.
> If the runtime thread pool is heavily loaded and only one core is available, they will only be concurrent, not parallel
Isn't that always true for thread pool-backed parallelism? If only one core is available for whatever reason, then you may have concurrency, but not parallelism.
