Oh I 100% consider it a fourth wall break and the afterword part of the story.
I don't think "lie" vs "truth" is a helpful frame here. Because when Borges puts himself in a story he's creating a character Borges and sometimes that character is the narrator (this isn't unique to him, "fictionalized voice of the author as the narrator" is a fairly common device).
In this case he's creating a character narrating the afterword who created a character coming up with and then writing Averroes and then when he stops imagining him, he disappears.
(And so does Borges, it is implied)
At a dinner, Averroes talks about the universality of common metaphors. As foreign to us as the North-West of Africa in the 12th Century, there still is something universal that we can relate with too, and those metaphors point us in that direction. I think it is more complicated than that in reality, metaphors change, but I do agree that we can relate to people who have lived long before us through what is left of their work.
I agree. And he also points to metaphors "growing" with time, because now they not only encompass the original meanings, but they incorporate the meanings (and connotations) of having been written by a specific author, in a specific time, and what those things suggest to us, centuries later.
Metaphors only die (or lose meaning completely) once we stop (re)imagining them.