There's a little bit more to it, however, since working memory is often a limitation for a wide range of tasks, and anything that could increase your short term

There's a little bit more to it, however, since working memory is often a limitation for a wide range of tasks, and anything that could increase your short term buffer to hold a few more items concurrently could help across a wide range of tasks, and potentially yield more "fluid intelligence." There are a number of conflicting findings about whether you can actually increase this, but the general concept is not quite as trivial as overtraining on puzzles as you laid out.