The embedded DCP file contains the same matrix I used above. It also contains a StdA color matrix, and forward matrices. Would be nice if RT used it, but would also be nice if RT used the right matrix based on the CalibrationIlluminant, which are in the incorrect (or unexpected?) order in this DNG file.
There are special case in RT code for dealing with DNG files which come from Adobe DNG Converter, these are there to ensure that the DNG files looks the same as its source raw file. I don’t know the details, perhaps @agriggio can shed more light.
I suppose the real answer to your question is that this is messy territory and no one has volunteered to shovel through it.