It's pretty clear he meant that he was holding the slide back, preventing the hammer from moving forward.
But would that always work on a Glock, which has an odd striker arrangement instead of a traditional exposed hammer, and doesn't have a way to decock it without pulling the trigger?
Holding the slide is not what prevents the hammer from falling. A semi-auto handgun with an exposed trigger that is placed out of battery has its trigger immobilized by mechanical linkage, regardless of the hammer's position.
Because of the way Glocks have their lug linkage, many will actually fire even when slightly out of battery. Knocking the slide back 1/10" and holding it there is probably not going to be sufficient to disengage the lug that prevents trigger pull.
Even if one could manage to pull the trigger with the gun significantly out of battery, I imagine that this would be a non-event with an exposed hammer because as you say, the hammer would fall onto the slide, not the pin. Still, firing out of battery would seem to be a function of the linkages, not striker-vs-exposed-hammer-ness.
But would that always work on a Glock, which has an odd striker arrangement instead of a traditional exposed hammer, and doesn't have a way to decock it without pulling the trigger?