"They could have achieved the same effect by implementing it in a language with continuations or coroutines"
They did. They just implemented their own minilanguage instead, that (AFAIK) simply has no conventional syntax-based serialization. It's still a language, though. Given their tight focus, it is completely plausible that they can create a little VM that will run far, far better than any general-purpose language VM could.
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"They could have achieved the same effect by implementing it in a language with continuations or coroutines"
They did. They just implemented their own minilanguage instead, that (AFAIK) simply has no conventional syntax-based serialization. It's still a language, though. Given their tight focus, it is completely plausible that they can create a little VM that will run far, far better than any general-purpose language VM could.