Anthony, In every system I have ever seen, you need addressable devices to work with an addressable panel.
It could well be that the addressable 'device' is actually the detector base and the detector itself is the same as detectors used on non-addressable systems.
When you have addressable devices, fire zoning becomes a function of the control panel software rather than a consequence of how circuits are wired.
So yes, the scenario you describe could be addressable, but I am amazed they are getting it to work on cabling from the 70/80's.
I would suggest that the easiest way to check if they are really addressable devices is to temporarily disconnect all or part of any of the original zone wiring and I would expect the addressable panel to 'notice' and 'report' all the individual addressable devices it now can't 'see', rather than a simple 'zone fault' condition that would indicate on a non-addressable panel.