The iceberg theory is used to comment a lot upon the subconscious of humans. It is estimated that we only see about 20% of the icebergs floating on top of the water, and that the majority of the iceberg lurks unseen below the water. So it is with our consciousness. We display very little of who we are as humans.
We can relate this theory to this brilliant story by refering to the character of Harry and how his daydreams and flashbacks reveal more of him than his appearance and his comments and actions at the time of the story do. In particular, one of the most memorable hallucinations that could be argued to express his subconscious is his flight of fantasy at the end, when he imagines he has been saved in time and he describes flying above the peak of Kilimanjaro:
...and then they were out and Compie turned his head and grinned and pointed and there, ahead, all he could see, as wide as all the world, great, high, and unbelievably white in the sun, was the square top of Kilimanjaro. And then he knew that there was where he was going.
Throughout the story, Harry seems to be a character who is eaten up by the way he has betrayed his artistic talent and sold himself out. The symbol of the gangrene slowly eating up his leg is a powerful metaphor for the way that his perceived failures eat up his psyche, and he himself equates the two, saying, "Rot and poetry. Rotten poetry." Even though he could be said to squander his gifts in his life, as he dies he returns to the purity of artistic creation. The beautiful scene of the peak of Kilimanjaro is a sight that creates awe in him because of its purity. Ironically, it is only as he dies that his artistic ability is able to fully manifest itself in creating a transcendentent experience that emerges from his subconscious. The iceberg theory therefore shows Harry for who he really is, in spite of the way that he has lived his life.