Comment by zbentley
Eh, not really. Async (in this semantic context) is generally about cooperative concurrency and also often about concurrent or multiplexed I/O. Pthreads aren't async by those definitions, though you can run async code within a given pthread as usual.
Goroutines are an unusual case, in that they don't have cooperative concurrency--they're pre-emptive--but the Go runtime does perform I/O using concurrent multiplexers under the hood.
So goroutines are kind of both: computation execution and code semantics look like pthreads, but I/O operations look like NodeJS on the backend.
Now, I'm not sure what "async runtime" means in the GP. If they're referring to I/O multiplexers, then they should say that. If they're referring to something else, then I'm not familiar with other uses of that term that would accurately apply to Golang.
Well, that's exactly what the kernel is doing when it swaps threads. When you block on I/O, you're voluntarily pausing your thread and doing concurrent I/O with another thread.
Async and threads are a lot closer than most people think. An OS is mainly a queue for swapping between async operations, and a collection of abstracted services that the async operations can request, like network or disk i/o.