First off, what year do you have?
My guess is that whoever installed it got a module to interface with the CAN-BUS system, allowing you to effectively double up the alarms. It is actually easier that way (and better) since only one wire was cut and a couple others spliced but your aftermarket alarm has full information about what doors were triggered and lock/unlock functionality. Further, if your car has a factory immobilizer that is also in use (that is good). In short, they likely did the job correctly.