Yeah, but how can he possibly know that Krypton will be destroyed in his grandson's generation? It's supposed to be Jor-El who discovers that -- and is disbelieved. I see the narration more as a figurative device than something that literally exists in-story -- like the Sunset Boulevard thing of a story being narrated in retrospect by a character who's already dead.
Perhaps it's narrated by his semi-sentient holographic avatar, existing somewhere in Superman's Fortress/ship.