I guess that Mike was speaking about DD THD that can't be downmixed if it's a mkv (it seems that core DD can't be extracted when DD THD is in a mkv, you can only get passthrough).
What the 303D doesn't do out of the box is multichannel downmix. For this you will need a patch to downmix DD THD/DTS HD to LPCM 5 or 7.1.
And for both things I believe that it's exactly the same thing than a D1...
About the 303D not being able to play BR rips with menu, he's right and that also the same thing on all new 3D models from all brands. But this wasn't a part of your initial question.