"On the other hand, users of FreeBSD don’t need to worry at all. FreeBSD already has a sufficiently good asynchronous interface for reading files, which you should use instead of thread pools."
Originally, as Igor has said in many talks, Nginx was written for FreeBSD, and supports what FreeBSD supports, and the Linux port has historically managed as it could. This is a case of actually adding something for Linux specifically, which is unuusual.
So the answer is it probably would have been implmented years ago if that was the case.