It's a bit better than green threading since out of order engines are imperfect at extracting parallelism and you can often use a second thread to fill in spare functional units that the first thread isn't using. There's also the danger that the extra cache pressure from two threads on a core will cause enough thrashing that using SMT will decrease performance on net. So it's more complicated but of course you're right that green threading is a much better first approximation than multiple cores. I suspect you already knew that but just didn't want to go into it.

One technique I've heard about is to schedule producer and consumer threads on the same core so that there's zero data transfer overhead and no extra d-cache pressure.