You are correct. Namor was one of the original 3 main title Marvel Superheroes along with Captain America and the android Human Torch (different from the Fantastic 4 Human Torch) and first appeared in 1939. After X-Men became popular he was retconed into being a mutant and has sometimes been called the first mutant because he was created first despite the in universe timeline having him be younger than Xavier and Magneto. When Apocalypse was created as a functionally immortal mutant from Ancient Egypt he took on the in universe title of the world's first mutant but the Submariner still occasionally gets the title out of universe.