Edit: I mistakenly read "OS-level threads" again where s/he said "application level threads" in the quote.

> For others, application level threads don't achieve the parallelism requires to efficiently use the hardware available.

You've got this backwards: OS-level threads execute in parallel (or, rather, may be executed in parallel), while green threads spawned within the same OS-level thread execute sequentially on a single processor core (but in no predetermined order).