The problem with that post is that you are using simple texture shadows, which makes it so that either an object is a shadow caster OR it is a shadow receiver.
But RTSS (or your own shaders) can instead use depth texture shadows, which solves this problem.
Depth shadows are needed for objects to know when the shadow starts, because as you may have seen with normal texture shadows, the shadow is actually shadowing things behind AND in front of the object, which is bad. So depth texture shadows solves all problems.
However, as I have no idea how to actually use RTSS (I instead wrote my own shaders for this), I cannot help you further. You would have to research a bit on how RTSS works and how to enable depth shadows for it.
You could start from here:
https://ogrecave.github.io/ogre/api/lat ... ow_mapping