NodeJS has more than one single thread, network IOs notably are on another thread. That's the beauty of it.

Why would network IO be on another thread? libuv gives node non blocking network IO (or so was my understanding).

Unsure about network (if it's true that it's on a separate thread, perhaps it's because network operations take cpu time even when readied asynchronously--e.g. even after you select/[e]poll to determine what events to trigger next, the actual read/write/sendfile/whatever ops do take a little time). However, filesystem IO is performed in threads in libuv on many platforms, due to the unavailability of stable async IO capabilities.