Interesting, so the idea is that your short-term/working memory is indicative of your fluid intelligence? Then increasing your working memory increases your intelligence.
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.
Actually it appears that increasing working memory alone doesn't increase intelligence (from the paper). But doing the 2 tasks together does seem to do the trick.
Seems reasonable enough.