The technique referred to in the OP is much more fundamental than NatureDemo’s trick: it’s just replicating geometry underneath the floor, and using the stencil buffer to mask out the parts that you might see around the mirror. Of course, if you’re in an enclosed room and the floor covers the entire room, you don’t even need the stencil buffer. (But if you do want to exactly replicate the technique, you can use a stencil buffer by creating a StencilAttrib.)
If you want to go the draw-the-scene-twice technique, the correct way to position the camera for a mirror is via a Plane, representing the surface of the mirror, and then applying plane.getReflectionMat() (along with the composition of the plane’s transform from the scene graph) to the camera. This is demonstrated in direct/src/showbase/MirrorDemo.py.
David