He's not got that traditional system.it's not at all an odd way to wire it, 3C+E is a fairly recent thing and before it's introduction most strappers were T+E, but then again most houses also had a single lighting circuit, so the neutral was already upstairs for it..
The permanent live for the landing light goes to COM on the landing switch. A T+E strapper runs between L2 & L2 to the downstairs switch, and then the switched live runs from the COM of that switch back up to the landing light.
erinues - what sort of cables run between the switches, linking the L1s & L2s, and carrying the switched lives back and forth?