What conclusion has Rainsford come to when he says, "Pure imagination"?

Rainsford says “pure imagination” after a fairly long conversation with his friend Whitney. They are on a boat and talking about an island off in the distance, which sailors call “Ship-Trap Island.” According to Whitney, there is a “curious dread” to the place, even among hardened and tough sailors. These burly men say that the place is evil—even the “old Swede who would ask the devil himself for a light.”
As they continue to talk, Whitney comments that he felt “a mental chill; a sort of sudden dread.” Here is where Rainsford speaks the words “pure imagination.” Rainsford is rejecting the idea that there is something ominous and evil about the island. Rainsford comes off a rationalist and, more importantly, a fearless man who loves big-game hunting. He even goes so far as to say that there are only two classes of people in the world: “hunters” and “the hunted.” What Rainsford really dismisses is anything that smells of superstition. Whitney, his counterpart, does not go along with Rainsford's logic and does not end up on Ship-Trap Island along with him.
All of this makes good sense. Richard Connell published the short story in 1924. He lived in a time when many tried to push to the fringes anything that was considered "superstitious." Even within religion, many scholars tried to push the "supernatural" away by rationalizing miracles and anything that did not have a logical or physical explanation.

At the beginning of the short story, Rainsford is having a conversation with Whitney on the top deck of the yacht when Whitney mentions that Ship-Trap Island has a bad reputation. Whitney proceeds to tell Rainsford that the ship's crew seemed a bit jumpy today, and he could tell that Captain Nielsen was anxious. Whitney then tells Rainsford that Captain Neilson told him that Ship-Trap Island has an evil name among seafaring men. Whitney then tells Rainsford that as the captain was speaking about the ominous atmosphere surrounding the wicked island, he felt a sudden chill as if the air was poisonous. Rainsford responds by saying, "Pure imagination" (Connell, 2). Essentially, Rainsford does not believe in the superstition surrounding the island and thinks that the captain and Whitney are simply paranoid. Rainsford does not believe the stories or reputation of the island but quickly discovers its true nature when he becomes stranded on the island with General Zaroff.