Just to be clear, what is the difference between a "module" and a "motherboard" by your definition? How does the "module" receive it's power without some kind of VRM mounted on a PCB?
The "module" is a miniature motherboard. Each one screws onto the risers of any ATX case and contains a chip, a VRM, a USB, etc., mounted on a PCB.
The Baby-Jet case has room for two of these 'mini-boards.'
All the mini-boards need to start hashing is an ATX PSU, cooling, and a controller (Raspberry Pi for BJs, basic 1U server for Sierras).
Sorry for the confusion, we are no longer going to call them "modules" or mistakenly suggest they are mounted onto anything except the case.