It is rather stunning that PCI signalling works despite this style of wiring, but the proof is in the pudding! Dealing with the wire inductance and the interference between the signals could easily be the hardest aspect of this implementation. Probably much harder than designing the logic itself. IIRC, the absolutely minimal working PCI hardware and software could actually be rather rudimentary, if one did not need implement the entire PCI specification in a compliant way.