They likely use off-the-shelf bluetooth modules which are already certified, and just talk some kind of higher-level protocol over it which sits above the SiG certification.
Many think this is true (even I did) but it's not. You have to certify the product anyway, even if you use an off-the-shelf module.
The only difference is that you can "certify with test" and "certify without test" (and price changes, starting from 5K a pop). Even evaluation boards and for prototyping, so I guess a end-user keyboard has to be certified[1].
