by 'chip' they mean...
once u start tampering with the engine like that you need a non-stock chip that has new tables for telling the ecm how to manage everything
cutting the cat off throws the second O2 sensor reading off (if it wasnt cut out too) basically you need a chip to tell the ECM not to take in account an oxygen sensor reading in its calculations... not to mention porting and otherwise allowing the engine to breathe better is probably going to mess up some calculations somewhere and you wont get the full performance from your engine compared to a custom chip