They are using WebGL. Using WebGL from WebAssembly has some nice advantages; with Wasm you can use languages that are more accommodating to the sort of densely-packed data that you need to send to GPU buffers.
Here’s an interview I did with a raster graphics editor that touches on why they use Wasm for GPU. https://digest.browsertech.com/archive/browsertech-digest-ho...
One way or another, the command buffers are an encoding process. They're not WASM code nor JS code, they're GPU code. You need to build them from one platform for another platform. There's no any direct correspondence between WASM and WebGPU, let alone WebGL.
Here’s an example of a Rust object with a layout that can be sent directly to a GPU buffer: https://github.com/paulgb/webgl2-glyph/blob/ef5a64a708856d67...
That doesn't mean it's a technical thing they're doing right, particularly.