Not a lie, but it's showing the magnesium shell. That shell is attached to a composite/stainless steel skeleton whereas the Nikon has a magnesium skeleton throughout. The lens mount, for example, is mounted to the composite mirror box. My suspicion is that the shell is largely structural, especially around the mount area as there's a lip on the lens mount that mates with the magnesium shell, but I don't have any good way to demonstrate that without taking my 5D3 apart. If you read Roger's article you can see how it's all put together.
Roger doesn't seem to say anything about it, that I can tell. A reader comment suggested the mirror box was composite, Roger indicated the circuit boards were screwed into composite, but there could be metal under it. The words 'magnesium' or 'steel' aren't used on the blog page.
It's interesting that the Canon mag-alloy frames seem more tightly constructed, using fewer pieces (at least, that's how it appears).
In other images of the 5DIII chassis, the 'mag allow shell over steel skeleton' isn't apparent, although it does seem the 5DIII's bottom plate may be steel (it's a different finish than the rest of the frame, at any rate).
So, I'm still not sure where you're getting this information...