To stay within the bandwidth limits it turns out that AMD is applying 4:2:2 or 4:2:0 sampling and thus shares red and blue color components to get to a lower bitrate over HDMI. The information itself is not exactly a secret, in fact AMD shared this information already during the Polaris launch. Hower AMD claimed that they supported 10-bit HDR gaming as well, and that is not right.

This is the opposite of what the heise article actually says. It says that AMD is NOT using sub-sampling for their 4Kp60 HDR output. HDMI 2.0 can do 4Kp60 at either 4:4:4 8 bit or 4:2:2 10 bit but not 4:4:4 10 bit. Despite their word AMD is actually using the 4:4:4 8 bit solution. Be careful as to not mix up bitdepth and sub-sampling.

This is basically the limitation of hdmi 2.0 it has nothing to do with the gpu itself