Which async web server should I use?

You’re correct. But what I said is still true for two reasons.

  • A lot of Go API wrapping libraries are written in Go and thus don’t suffer from the issues where OS threads become blocked (whereas a number of Julia API libraries are wrapping C API’s which do block the OS threads)
  • Go can create new threads at runtime, so even if all your OS threads become blocked, Go can create new ones (though with a significant performance decrease).