I wonder that too. On the site they only list a few Epson models and some Canons. Many are bubble jet printers, which I don't think you can even buy anymore.
Can you buy a empty HP cartridge and their refill kit and do it yourself?
Right now you can only get edible ink cartridges for Epson and Canon printers. I guess HP hasn't parted with their tank technology so a third-party doesn't have rights to it yet.