Sorry for the confusion.
Two options are available for an exhaust manufacturer when creating a true dual exit exhaust system: you can either design a center canister with one pipe leading out to each side of the car, or you can create two canisters so that each side of the car has it's own dedicated canister and piping.
Here are photos of each type of design for an E9x M3 application, just for an example:
Dedicated piping and canister for each side of the car.
Center canister, with pipes going to each exit
The 135i does not have the available space below the car to house either of these options. If you wanted to use either of the above choices, you would have to cut out the storage well in the trunk area, weld in flat sheet metal in it's place, weld exhaust hangers, and remove the charcoal canister located in the rear of the car.
That process involves far more custom work than people are willing to accept, and Eisenmann provides a simple bolt-on solution instead. Eisenmann's piping is routed to bypass all of the offending OE equipment and direct exhaust gasses to the passenger side of the car.
I hope that explains why the "dummy pipe" compromise is made. It's technically not an ideal solution but is a better choice for most people than the thousands of dollars of labor required to do the job using two canisters.