I would assume you are right, since he does mention needing four of the 4' linear rails. There's clearly an axis missing.
I was just too lazy to model the other half of the Y axis haha. It will be a mirror of the other side anyway
This is the current state of my design until I get some details worked out like the wiring tray, electronics housing, etc. I still need to make a parts list and start searching for what to buy