Yes, I'm also pretty sure it's wood - the prototypes clearly has it, unless it's the worlds best, and unique per unit, imitation. Look at the pictures towards the end of that Gearslutz thread. Being an Axel Hartmann design (always classy), I'm confident that no plastics are specified in the chassis construction.
The Keylab and MiniBrute gets some flak exactly because it's not solid wood, but veneered fibre board or something (still waiting for confirmation of that), which would technically be wood, just not in the form people expect. For the MatrixBrute, Arturia does not specify the materials on the product pages, in the press release or any of the blurb on the various shops web pages - still they get hammered ahead of time for "breach of trust".
Whether it's deserved or not, Arturia needs to step up and start building that trust, since the hammering is on, as mentioned. They're not interacting much with the "fans" and the community who are the customers, unlike other boutique'ish manufacturers, and in the mean time outspoken people are falling over each other to take swings at them over a seemingly trivial issue.
IMO, with the Origin and now the MatrixBrute (possibly not the best choice of name, but that's a different issue), Arturia is making some of the most desirable synths. Personally I wish that instruments like these will succeed, and that depends not just the quality itself or the specific materials, but also how Arturia is generally perceived among the high-end synth buying public.
