"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.
How about a change in diet only when reaching mature size (the rock eating suggested in the article)? Doesn't seem too farfetched, and I think it would allow for growth when still developing.
"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.