Did CS Lewis believe that Out of the Silent Planet was a true story?
Lewis was trying to convey the Judaeo-Christian story to a modern audience. He imagined humans created on Venus, Earth, and Mars. On each planet, the Devil tempted humans. On Earth, humans had yielded to temptation and done what was forbidden. On Earth, it was the fruit of the tree of life. On Mars(?) it was an island where they were forbidden to spend a night. In each transgression, God wanted to save humans. On Earth, the price was high: The sacrifice of His only begotten Son. Next time, the price would be even greater. Lewis never says what that might be. So: No. Lewis never believed that the “interplanetary problem” was a true story. He was trying to tell the Bible story to modern people.