How does the phrase "the meaning of an episode was not inside like a kernel but outside" relate to the story?

Here Conrad points out the difference between Marlow and other seamen. Marlow wasn't interested in merely telling stories, rather he looked for truth. He looked for truth outside the "kernel" of his experience and comfort zone. He saw things for what they were. In the Congo, for example, Marlow saw the depravity of the whites in all its horrid detail.