They probably needed the review to release the kernel and mesa driver parts. There is new UVD firmware (215 KiB for one chip, previously the biggest file was 32 KiB) and updated RLC firmware, maybe the availability of much more code will make reverse engineering easier.
DRM isn't their only argument for not releasing sources nor documentation of this firmware, the resources needing for the legal review of the documentation are another reason that they state.
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