I think async vs threads is about completely different trade-off. Nowadays all operating systems so preemptive scheduling, but green threads (and all async by the extent) use cooperative scheduling. I believe most of discussion here is actually about pros and cons of those scheduling models.

one exception is I think cancellation model, but I'm only aware about rust that does it that way, all other runtimes will happily run your green thread until it finishes or cancel by itself similarly that you do with synchronous code.