apparently having all the logic inside firmware (like Nvidia does)
Based on this part of the quote, the nvidia implementation has a lot of the functionality inside not open source binary firmware blobs. And that includes the functionality that the HDMI forum wants staying secret. It’s in the closed source firmware, so this is ok, since the open source part only has to send instructions to the firmware, and not include the implementation.
AMD has less functionality inside the firmware. Which means the drivers are “more” open source. But any proprietary stuff that the HDMI forum wants staying secret would have to be in the open.