I am not quite sure but I do believe Purism actually designs their own hardware. They are pretty open about hardware selection, which would be weird if they only sold branded Clevo.
Besides the case matching perfectly with the Clevo barebone of the year (except for the hardware switches, which are in a plastic bezel on the display hinge, so that's a custom/modified part), there are teardown pictures online and the innards perfectly match Clevo machines as well.
But Purism doesn't design these, Clevo does. Purism pretty much only chooses the anodization colors and stickers.