how do you suppose these observations relate to this novels main topic africa?

Much like the Belgians, Marlow thinks the Roman who ventured into the wilds of Britain wanted quick money, promotion or to fix some kind of debt,

"Or think of a decent young citizen in a toga—perhaps too much dice, you know—coming out here in the train of some prefect, or tax-gatherer, or trader even, to mend his fortunes..."

THe Romans had once thought of Britain (dark place of savagery) as the Belgians now think of most of Africa.