What she sees is an illusion - a trick of the light. It is clear that there is no distorted man and, most importantly, it is clear that Chielo really is the same friend from the marketplace. Here, Achebe is not meaning to say that Ekwefi is gullible or easily scared; he means to say that in dark isolation, fear overcomes one's understanding of truth and humanity.
This same fear incites people to violence. When Ekwefi steps on a twig, snapping it, Chielo turns and screams "there is somebody walking behind me! . . . Whether you are spirit or man, may Agbala shave your head with a blunt razor! May he twist your neck until you see your heels!" (TFA 74).
It is fitting that Achebe's characters only see dark figures when they are alone at night, because Marlow is alone throughout the book, and seems to often be narrating during the night. He is not met with faces, but with carnal masks of his own superstitious design. Indeed, the people Marlow sees are not people, they are "atrocious phantom[s]" (HoD 135), "dark human shapes" (HoD 136), "swift shadows" (HoD 137), or, often, "figures" (HoD 93), which "mostly black and naked, move about like ants" (HoD 79). They sound like animals, monsters, ghosts and demons. This passage