Isn't the point of this repo to compare the usability of different developer tools?

Not really. The Readme says this:

"This repository contains multiple implementations of the same 3D scene, using different APIs and frameworks on various platforms. The goal is to provide a comparison between multiple rendering methods. This is inherently biased due to the variety of algorithms used and available CPU/GPU configurations, but can hopefully still provide interesting insights on 3D rendering."

It's a mixed bag of APIs, software, hardware and tools, approaching the rendering in very different ways. There's even an offline renderer (Blender Cycles) and a software real time renderer in the repo.

> The goal is to provide a comparison between multiple rendering methods.

Well, isn't usability the most important criteria to compare it on?

No, one of these methods takes seconds to minutes for a frame to render. One takes a lot of CPU and a bunch are letting the GPU doing the work. Not to mention that a few examples run och very specific hardware only (PS2, GBA etc).