He talked about that in the article. His point being that metal bones would be effectively indestructible for typical loads placed on them by organisms and if they ever received damage, they would bend instead of break.
"So it seems that there are no fundamental reasons why animals could not evolve a metal skeleton"
The main reason is that most multicellular organisms (especially vertebrates) go through developmental stages where they grow in size. Mineral bones can be grown like this, while metal ones probably could not.