Yes, that's exactly what it is. But it also matches wikipedia's difinition of coroutine: "Coroutines are computer program components that generalize subroutines for nonpreemptive multitasking, by allowing multiple entry points for suspending and resuming execution at certain locations."
Fibers and coroutines are different uses for the same thing. If you schedule your coroutines based on the kernel scheduler, they're fibers. If you jump between fibers explicitly, they're coroutines.
I think it's mainly a fiber library, actually. The TCP and channel functions yield and resume automatically based on when the underlying operation blocks.
They have yield(), but I don't see the opposite, the ability to explicitly resume a stopped context.
Edit: for a "pure" coroutine library, I'd recommend libco from BSNES. It only has 4 functions: active, create, switch and delete. I wrote a fiber library using libco plus libuv, which was fairly easy.
