The microwave may be configured at time of installation for one of three ventilation types:
A. Outside top exhaust (vertical duct)
B. Recirculating (non-vented ductless)
C. Outside back exhaust (horizontal duct)
So, yes, recirculating (non-vented ductless) configuration does not require a vent going to the roof, or any venting to the outside at all; the steam and cooking smoke are directed to a charcoal filter, then out the front, through the grille over the door.