Because it not a helpful comment. It's wrong in a lot of ways and misleading in others. Like dragonwriter said, having coroutines in your language or a fiber l

Because it not a helpful comment. It's wrong in a lot of ways and misleading in others. Like dragonwriter said, having coroutines in your language or a fiber library is not close to the same thing as the language having a scheduler that maps those fibers onto native threads for you. Some of the languages mentioned can't even execute multiple threads on multiple cores in parallel.