Actually, the book mentions the platinum has to be magically treated to make it as sturdy as steel, so it very well might be pure platinum.
And that just begs the question of why one would bother making armor out of the stuff. You get two points of cold and sonic resistance, but the trade-off is an inversion of mithril's benefits.
But being worse than useless doesn't make something dysfunctional, so I digress.