Interesting, I heard that once before but just happened to forget it.
What I find perplexing, however, is that if I was to move the Water Closet to the left in the diagram all the way to the stack essentially, this would take care of the "horizontal vent" problem. For that reason, I don't get it.
I'm starting to think that I need a new stack in the house. Unfortunately, if I put it right above the sewer main, the stack would come out on the roof about 3.5 feet horizontally from a window and probably 3-5 feet below the highest point on the roof line (which I think violates UPC).
Any thoughts or advice would be appreciated.