From what I understood looking at the photos in the article, "relays" are some fancy-looking external box. It is quite possible that those are "pulse relays", i

From what I understood looking at the photos in the article, "relays" are some fancy-looking external box. It is quite possible that those are "pulse relays", i.e. RPi sends a short voltage pulse to switch relay on, then another short pulse over the same wire to switch off. From there all you need is two (or more) diodes to handle multiple pulse inputs. Some pulse relays even have diodes built-in (i.e. can handle multiple inputs from the get-go). You can easily spot this sort of installation - wall-mounted light switches only go one way, and return to original position as soon as let go. And each press changes on/off state - sort of how Caps Lock works.