Fibers and thread pools are not exclusive - you can schedule fibers onto threads. Nginx already uses event-based state-machine approach so context switch is even cheaper than with fibers (at the price of ugly code).

What they talk about in the article is that they offload potentially blocking operations to thread pools. That's not about networking, which is already done asynchronously, but about e.g. reading from a file not present in the page cache. Note that detecting that the needed bytes are already in the page cache and thus the read wouldn't block and should not be offloaded is tricky. As far as I can see they haven't done that yet so this new feature (edit: not so new, the post is from 2015) is not without downsides.