Answering my own question:
Lambertian diffusion is calculated per vertex basis, i.e. illumination is calculated based on a vertex normal and the direction towards the light source and then interpolated thru baryonic weights in the fragment shader.
In my example, there are no subdivisions on cube’s faces and the only 4 vertices are at the corners of each face, so no details from Lambertian diffusion are visible in between.
Adding subdivisions solves the problem:
new THREE.BoxBufferGeometry(80, 100, 50, 8, 10, 5),
instead of
new THREE.BoxBufferGeometry(80, 100, 50, 1, 1, 1),
https://jsfiddle.net/tfoller/9gp3z5f2/43/
This leaves ugly wireframe-like artifacts at a close distance due to quad division into independent triangles but I believe there is no easy solution for that.