Engraving can mean that the printed area is engraved (intaglio, recess printing, line engraving) or the non printed area is engraved (typography, surface printing).
With German stamps, setoff appears when a paperless print is made and the ink is deposited on the base plate.
I think that is not completely correct.
It can occur this way, just as it can occur when printed sheets are stacked and the interleaving was forgotten. Offsets also occur on photogravure stamps that are not printed from plates but cylinders. Even since they have been printed on the web, offsets occurred. Full offsets are likely to be caused the way you described. Not all offsets are full offsets.
Looking at the stamp against a strong light source will tell if both sides coincide. If they do, the odds are stacked against it being an offset. If they do not, you might be correct that ink was deposited from the printing plate on the base plate.