I believe you’re correct. DACs obviously can’t determine volume at all, but amps can try to use the impedance to create an estimate.
This probably isn’t accurate though. If you really want a good estimate, you would have to calculate it with current voltage output and the specs of the headphones/IEMs in question.
I’m just a hobbyist too, but my headphones are extremely inefficient so I’ve spent some time looking into this. Too bad we don’t have oratory here