Not quite sure that it means they are rare. Rarer than what's offered now, maybe, but perhaps not rare in general. It's out of production, so we wouldn't see it as often as the stuff that's still being sold (ie. the Slate and Emerald Slate stuff). There were a lot of them available when they were out, though.
It's tough to judge rarity on newer items (esp. those that we don't know the production #s to) when product gets moved from the Sellers' hands to the consumers, as the consumers tend to recycle product less frequently than dealers.