Found it! You have to bind the VAO before attaching it to the VBO. I told you it was one line of code.

27-instancing/shaders/27-framebuffers.vert.glsl

@@ -21,6 +21,7 @@ void main()
 
   // THIS DOES NOT, see Scene::setup_instancing and Mesh::draw
   vec3 in_offset = aOffset;
+  // in_offset.x = gl_InstanceID;
 
   vec4 offset = vec4(aPos + in_offset, 1.0);
   gl_Position = projection * view * model * offset;