I never said anything about density, I just quoted it. If you cut into enough aluminum castings, you'll find those voids. Some in honda TB's are big enough to fit an actual rice grain into. You put holes in aluminum to lighten it, not strengthen it.
Temperature (heat treating, casting temp, etc), gas exchange, and alloying can drasticly change the structure of the metal itself. Look at cast iron. The difference between a Sikorsky rotor hub and a ordinary park bench are just a few simple tricks. At least that's the jist of what their engineer's memo told us.