You'll notice the function's documentation is "Schedules a fiber. The function must be called on a fiber." so while it may not be a literal difference (after all they're both semi-user space), they are difference systems to use, but they're both roughly at the same level of "power".

If you're not using a scheduler with fibers then I'd argue you shouldn't be calling it a fibers system (and rather coroutines), since otherwise it's a distinction without a difference.