Anyone who's taken fundies want to weigh in on this bouyancy class? My understanding is that if you can hover w/o finning or sculling with your hands, then your bouyancy control is good.
The hypothetical diver you describe may or may not have "good" bouyancy control (as measured in DIRF). Personally I define "good" as plus minus 2 ft of intended depth in midwater. With no sculling of the hands and the ability to maintain team positoning. If you look at the DIRF standards you'll see that this is similar to what GUE expects.
IMO you don't need to be "good" or "excellent" to enroll in DIRF. Shoot that is part of what the class is for, to improve your bouyancy skills. You do need to have enough bouyancy control going in to not be frustrated by the other skills and derive a net benefit overall. E.g. If you are constantly corking to the surface its hard to share gas or shoot SMBs.
Personally I would enroll in the bouyancy class. Its staggered over several weeks to get some time for stuff to sink in. Its personal, (hopefully) convenient and cheap. It has everything going for it and I don't see a single drawback.