Smaller web servers have been event based since they first showed up, it is the natural way to build them. There was no epoll() available, but a select() loop is pretty much the same thing. The super useful thttpd had been around a long time at that time.
What caused people to start using non-forking web servers for regular public web applications was probably that PHP finally got a stable FastCGI mode going about ten years ago. Because, honestly, most of the web is PHP and it was even more dominant back them.
And when people posted their web server configs in forums at that time, nginx configs were much more readable and concise. The easier config format is what caused its popularity to soar, not that it used a particular syscall half a decade earlier.