That isn't the case here. Did the read the blog post? Toward the end it also says, "The Raspberry Pis Ethernet port and 2 USB ports are available on the back panel of the Baby Jet for you to connect to." So there was no confusion between 'Serial' and 'USB'.
Yes, I've read the blog post. Baby Jet may have maximum two mini-boards, each connected through one of the two available USB ports on the R. Pi. Simple solution, battle-tested tested through
MiniPeon project and perfectly clear.
It doesn't say anything of the sort in that blog post it says the USB ports are free. You're missing the point. My issue is the lack of clear and accurate information. We can infer it's USB now from the postings in this forum but what's so difficult about writing a concise overview of how it is all going to work? What about releasing a diagram of the mounting hole spacing for the 'mini-board' and what kind of mounting assembly will be required if we want to try alternative cooling options.