Based on what you describe here, I'd say your comparative understanding is about two decades out of date. Async I/O has been a capability in various Unix and Unix-alike kernels for that long.
As in signal based AIO? Have you ever tried to use it in a high performance network server, where you want to have reads also satisfied from the cache if possible?
Because that is like pulling teeth on UNIX. See: https://groups.google.com/forum/#!topic/framework-benchmarks...