The problem with the people he says he interviewed is that none of them had any reason or incentive to tell Wilson if the claims were true.
They all would have been or would be in compromising positions if the claims were true, because they would have been involved in the transactions themselves, in the past or at the time Wilson talked to them.
Africa, in spite of its riches and due to corruption is a continent filled with corrupt governments and many underpaid and corruptible officials. For Wilson to take their answers at face value shows either he is really stupid, or he did it just for show, not actually believing them himself.