I think that observation may be the key. It's hinted at in the study, in fact. These kids had UNEVEN subscores... and I have to wonder at their performance when looked at over several unrelated areas. Are they chess prodigies that also write poetry like college students at age 8? Probably not. If they write like typical 8yo peers, in fact, that might be a clue as to what makes prodigies so different from most PG people.
Perhaps that very quality of being 'omnibus gifted' in the historic sense of the term-- that is, having global multipotentiality-- may prevent the development of an area of prodigious ability.
There is no way that anyone would term anyone in my family a prodigy, either. We're almost all jacks of all trades, so to speak, though we frequently appear to be "masters" to those around us, in whatever discipline we happen to be working in (at least as adults, but we also see this in DD with individual subject teachers who think that she IS working in an area of 'profound ability' when it's just another class like any other). We tend to have the ability to be "experts" in multiple areas of not-particular-strengths or interest, and maybe even 'exceptional' in areas of interest and extra effort. Not "prodigy" or "superstar" quality, however. But I suspect that it is because of the relative evenness of ability throughout a wide range of subdomains.
Working memory may be only a small part of the story there; in those with multipotentiality at play, it may simply allow you to work in parallel and do more different things well, not to do any one thing to such an extraordinary degree.
The multipotentiality may even get in the way of the kind of focus needed to develop prodigy talent/ability by 'diluting' the effort across unrelated domains.
