I'm just going by the original question in the other thread at
http://forum.x-cart.com/showthread.php?t=21990 -
"Anyone know of a way to exclude the products of 1 specified main category (and its subcategories) from ever displaying in randomly generated Recommended Products results?"
I have not tried it, so I don't know if it works, but it does make sense that if a main category is excluded that any subcategories of the main category would be excluded also. Another thing to consider is that the thread was from 2006, and the version listed are 4.0.x branches. There may have been changes since then.