It has support for AIO, as had vim for quite a while, but it's not pervasive in the standard library and the ecosystem. Both editors have way too many plugins that block the UI needlessly while you're typing (autocompletion, syntax checking etc).
It's quite more popular than you think. `M-x term` is an example. It's a full-blown terminal emulator in Emacs that runs entirely in another process. Here is another example of async grep/ack/ag: http://tuhdo.github.io/helm-projectile.html#sec-9 . As you can see, Emacs gets line by line results from grep/ack/ag without blocking and gives you new results and you type.
Or Emacs's fantastic support for communicating with REPLs in many interpreted languages.
