It's actually not that unreasonable. Lots of case manufacturers have cases ready on day one for new iPhones too. They are either given advance dimensions (big companies like Belkin who Apple carries in their own stores), or they just take dimensions for part leaks and hope for the best. And Apple doesn't just send review units, they also send units to accessory makers. For example, I e-mailed Startech a while back to ask about compatibility and they said they were "currently testing" with the new MBP; and this was a week before anyone had theirs.
So it's not too crazy to think that there could be companies already shipping or at least accepting pre-orders on cases. There's incentive to be first-to-market with some of this stuff; so these companies have learned how to turnaround a product QUICK.