Nearest I can tell what the Marine's mean when they say organic CAS, is really Marines doing the flying rather than landing in the field.
The STOVL thing has more to do with flying them from Amphibs than landing in the field. They argue that we might want the ability to move a MAGTF into a region without commiting a CSG, what conflict would require an LO fighter but allow successful RW employment without SEAD is beyond me however.
I guess if you follow their logic, they want a STOVL aircraft and an LO aircraft but their requirements don't neccesarily drive them being merged into one airframe.
I still don't buy the amphib requirement, but whatever.