What exactly does this mean? Isn't it just a case that the driver wouldn't be HDMI-certified, or are they actively prevented from distributing the driver?
My reading: AMD signed a contract with the HDMI Forum in order to get access to the HDMI specification and be allowed to create HDMI products (and use the HDMI trademark), which includes an NDA regarding the specification. An open-source driver would violate that NDA.