It's not retconned. Selene has been shown in several books (X-Men, Avengers) to have been born 17,000 yrs ago in Hyborian Age. Well before Apocalypse was. Given the lack of technology and means of communication it's likely they never heard of each other for centuries, especially given Apocalypse was prone to being more visible and then sleeping for hundreds of years.
Neither. Apocalypse has been given (with good reason) focus and prominence in Hickman's X-World. I think the reason why many would assume Apocalypse is the first and older is that while Apocalypse made himself known throughout the world by his "survival of the fittest" and promotion of mutants, Selene kept herself off the radar to feed and not have a global vision. But I agree I would love to hear the two of them discuss their perspectives in the thousands of years they have been alive.
Apocalypse was probably the first mutant to be identified as a mutant. Selene in her time must have been indistinguishable from just another evil sorceress.
Like Namor it is sometimes called the First Mutant because it was the first to appear publicly in the 20th century.
This is what Tini has to say about it.
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