This is a bad explanation because it doesn't explain how the concurrency actually works. Is it based on stacks? Is there a heavy runtime? Is it stackless and everything is compiled twice?

IMO every low level language's async thing is terrible and half-baked, and I hate that this sort of rushed job is now considered de rigueur.

(IMO We need a language that makes the call stack just another explicit data structure, like assembly and has linearity, "existential lifetimes", locations that change type over the control flow, to approach the question. No language is very close.)