Way back when that was written, I had spoken to someone from Journe who said there was a problem with the way the crowns were made, where they oxidized internally (or had bubbles or something like that). This caused the crowns to break. So F.P. Journe took a bunch of crowns and started snapping them and looking at them under a microscope to find out the problem (which he did and corrected). This was a remarkable thing because the crowns were made of platinum and therefore expensive.
Now... whether I misunderstood the story, whether the crowns were once made of platinum, whether they tried making them in platinum and never did, who knows? I defer the most up to date info to someone who currently works at Montres Journe and has recently shared that the crowns are in white gold on the platinum pieces.