But one of the reasons you're able to process more with async I/O is bypassing the costs of the thread-per-connection model. So you're not forced to store all the thread's stacks and you don't have expensive context switches.

As the parent illustrated, even with Go using nonblocking I/O, it's perceived benefits in that area isn't that great because Go still semantically has a thread-per-connection. So the performance characteristics of Go isn't simply async vs sync I/O.