The interface module is only connected to the “high” side of the zone on the alarm panel so it’s not creating a parallel circuit since there’s no loop back to ground/common. It also doesn’t draw any current from the zone.
Basically it uses an comparator circuit to compare the voltage coming in from the zone to the baseline voltage from the panel’s AUX. The tunable potentiometers are used to tune in to the threshold between open and closed.