Laure shared that earlier, she scattered foliage through particles, but it didn't match the baked lighting look, so the new tool is combining the best of both approaches.
In the shader, the leaves are a mixture of rotating towards the camera and rotating towards the normal. Additionally, there is an automatic setup in the tool that generates a low/mid/high-density mesh and cross-fades between them. However, you can make the foliage as dense as you like, with a noisy texture with lots of leaves allowing a lower density.
According to the author, the mesh generation happens in script, and she bakes in the normals, light, and color info that is then used inside a billboarding shader. It can be used with any mesh, not just plants.
