It's not about the callback at all, the example is poor and the explanation is lacking. In Go, many standard I/O functions use async I/O under the hood. When you call such a function (inside the handler callback, that is), the current goroutine is suspended yielding to other goroutines, so waiting for I/O doesn't block the OS thread inside the request handler. It's basically what you'd do in C# with async/await, except it's implicit and you don't have to mark functions as asynchronous manually. I.e. you write simple synchronous code and Go makes it asynchronous under the hood. There are several disadvantages, though: it's not tunable, and if there's a CGO call (call to a C func) which uses I/O itself it will block an entire worker thread because the Go runtime isn't aware of any of it.

As far as I know, Java's new virtual threads have the same idea.