Nginx has an evented I/O architecture. libev/libevent implement evented I/O but evented architectures have got nothing to do with libev/libevent per se.
nginx uses native evented I/O provided by the operating system (epoll on Linux, kqueue on *BSD, /dev/poll and event ports on Solaris, input/output completion port on Windows).
libevent and libev provide abstraction layer for those mechanisms and unified interface for all supported operating systems.
> native evented I/O API provided by operating system
I know it's just semantics but I recently learned that some people actually believe that synchronous I/O API provided by OS is actually… well synchronous I/O. Obviously there cannot be any synchronous I/O going on in any modern operating system.