retaining wall as hollow inverted right triangle

Sure ... like you don't have landslides in the US?
That article theory assumes no changes brought by the one thing you can not stop ... water.
Not to mention earth tremors, tree roots, animals diggin, trucks parked ... mm ... yes, that will do ...

I like this bit

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

• Hammer
• Tape measure
• Level
• Rubber mallet
• Spade
• Utility knife

Compactor, If you build a timber wall, you'll need a circular saw to cut the timbers to length.

Lots of people think a retaining wall needs to hold back all 6 gazillion tons of soil in the yard behind it. It doesn't. It only needs to retain a wedge of soil, or elongated wedge of soil, similar to that shown in Fig. A. In simple terms (our apologies to all you soil engineers out there): Undisturbed soil—soil that has lain untouched and naturally compacted for thousands of years—has a maximum slope beyond which it won't “hang together” on its own. This slope is called the failure plane. If left alone, the soil behind the failure plane will stay put on its own

I think the builders at Thredbo must have used this manual when they built the lodge.

the soil on a slope slides down due to gravity and failed cohesion in the soil itself. This happens at a continuous rate on any land that has any sort of slope. THe only question is at what speed this happens so it is a matter of when not if.
A perfectly flat land that has a step in it with a retaining wall also falls into the category of slope since there is no soil on the other side of the wall and gravity will cause the soil on the high side to flow against the wall. The wall needs to replace the missing amount of soil and so needs to be built accordingly. To think that it only need to hold up the loose bit of soil described in this article is a joke and ignores the most basic principles of building.
You build a flimsy retaining wall at your own peril. And may be for some time it may even work, but eventually nature will push it over like a wall of cards.