The source code is not a clean room re-implementation of the the HDMI 2.1 spec and as such contains IP owned in one way or another by the HDMI Forum. This would almost certainly preclude them from distributing source code to non HDMI Forum members under any license according to what I could glean from the article.
Ah, thank you, this is interesting. So in a way the original IP is kind of poison which forever taints their source code, diminishing what they can do with it.