It would definitely make sense (the more you can hold in memory at one time, the faster you could learn stuff), but I don't think they've actually concluded tha

It would definitely make sense (the more you can hold in memory at one time, the faster you could learn stuff), but I don't think they've actually concluded that's how it works. They are clearly related though. Another hypothesis is that working memory and fluid intelligence both utilize the same neural paths/network, so an increase in one benefits the other. The study itself is linked in my article above if you're interested in reading it. It's pretty interesting stuff.