If the player's original momentum didn't take him into the end zone, then the momentum exception cannot apply (8-5-1-a Exception, part b). So now we simply have a fumble into the end zone. Who is responsible for a fumble? Just because Team A knocked it out of B23's hands doesn't mean they are responsible. 8-7-2-a says that whoever fumbles it is still responsible for it until a new impetus is imparted. So Team B is responsible for a fumble into their own end zone. That gives us a safety.