As Robert Hampson notes, the comparison between Marlow and a Buddha is partly ironic and partly straight. On the one hand, if Marlow is the possessor of knowledge, it is knowledge of darkness rather than of enlightenment. On the other hand, Marlow as a story-teller can be seen as offering a teaching-tale comparable to the Buddhist Jataka.
H. C. Brashers, 'Conrad, Marlow and Gautama Buddha' (1969)
Peter Caracciolo, 'Buddhist Teaching Stories and Their Influence on Conrad, Wells and Kipling' (1986)
William Bysshe Stein, 'Buddhism and the Heart of Darkness' (1956); 'The Lotus Posture and Heart of Darkness' (1956); 'The Heart of Darkness: A Bodhisattva Scenario' (1969)