your tuner is right. one of the wires either connects to the rpm or the injector duty wire from the ecu.
connecting the wire to monitor injector duty was just that, monitor only. but if you connect the wire to the rpm, you can also use the other features such as gear based boost or different boost levels according to revs, as well as monitor revs. mines connected to monitor revs but i dont use gear learning, gear spcific boost, etc.
the part where you can adjust duty cycle is nothing to do with the reading of injector duty cycle.
eg/ mines set to 1bar with a duty cycle of 50%. at full throttle, this keeps me at about 1.00bar with small spikes no more than 1.05bar. putting duty cycle to 48-49% only gets me boosting a max of 0.95bar.