It may seem counterintuitive, but one reason wood works so well, particularly for tops, is that there is a lot of air in it. In particular, softwoods tend to have more open space in their structure than hard woods. This makes them softer, but also stiffer for the given weight. One of the few hardwoods that has even more air in it than most softwoods is balsa, which gives balsa parts of the same size a higher stiffness to weight ratio than spruce. Of course, you give up something for that; usually surface hardness. We see this all the time; Western red cedar is often lower in density than spruce, and can make a very light, stiff top, but we all know how easy it is to dent.
What many people don't appreciate is that strength is seldom a limiting factor in guitar construction, but stiffness is. In general, you can use a low density top wood and leave it a bit thicker to get the stiffness you need without any problems, and it will generally be more than strong enough. The 'sandwich' tops are a case in point. They consist of thin skins of normal top wood, usually about .5mm thick, glued to a honeycomb core material who's sole purpose is to produce an air space between the two skins. The core has no real stiffness itself, and every effort is made to reduce the amount of glue used to a practical minimum, since any excess beyond what is needed to reliably stick the sandwich together is just dead weight. The result is a top that is about 40% lighter than a normal top, and easier for the strings to move, but just as stiff. Even though it has only half, or less, of the wood in it that a normal top does it's still strong enough (if you're careful).
You can see from this that stiffness depends at least as much on the structure as the material properties. For a given material the thing that predicts how stiff a given size piece will be is the Young's modulus, and in engineering discussions that would be the number given. When they talk in that article about the stiffness being 20 times higher they don't say whether that's for the same size piece, or whether it's the Young's modulus that is three times higher.
If the same size piece is 20 times as stiff, and 3 times as dense, then you'd need to make it 1/3 as thick to end up with the same weight. This reduces the stiffness by a factor of 3 X 3 X 3=27, so it's not as stiff as the normal top at the same weight. You get pretty much the same answer if you assume they're talking about a Young's modulus that's 20 times as high.
One can see uses for a new material like that where structural size is a factor. Really tall buildings had to wait for the introduction of structural steel; brick walls got so thick once you went beyond eight or ten stories that they became impractical. The pyramids in Egypt are huge, but there's almost no open space inside them. The Romans pushed concrete structures like the Pantheon to greater heights in part by incorporating volcanic tuff, basically foamed rock with lots of bubbles in it, as 'filler' in the structure at the top. Even so, it's a massive structure, which is one reason it has survived for a couple of millennia in earthquake country.