Th palace press office has a long history of stretching and all out misrepresenting the truth. Here you need to read what they said closely.
They said it belonged to the historical house...yes the Grimaldi house is historic. They implied nothing about the age or length of ownership. The only noted who owned it.
More likely this piece is very recent, given to the palace or Charlene on a Tour and therefore to her in the position of Princess and not a personal piece.
It would have been easier to say so but it would have brought questions. Things like who donated or gave it and therefore had spent so much. Monarchs of recent are being more transparent about those who have and buy influence but Monaco is not. It also would have brought criticism with some of the people of Monaco who are still having financial issues.
In short, easier to imply and let the bloggers and fans think otherwise. It is very clear this is not an antique piece. Even expert bloggers on the royal jewels board have noted this.
Until Monaco and it's press office learn to be more transparent, read between the lines for their 'carefully worded' releases.