Well, put it this way: the equivalent passage in Conrad is most definitely describing the experience of the Romans in trying to settle Britannia as a province, rather than the voyages of Pytheas, as it were; and the aptness of the passage in either version depends on its applicability to the contemporary situation of the Belgian Congo, which is again one of imperialist colonization. So if Marlow is trying to make us think of explorers rather than colonists he has rather missed his own point.