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.