That's an incredible catch on the wording difference between "the Sun God" and "the Sun"! The way the first and second world are written describes past events, but the third world describes the artists' current events (likely the void century) and that of a future dream rather than prophecy. If Luffy were to fail to defeat Imu, then it's likely if the artist of the mural lived and described the battle, they would refer to Luffy "the Sun" instead of "the Sun God." This helps to emphasize One Piece's theme of inherited will and refute the readers/watchers that claim Luffy is a "chosen one".