I think Bungie is hinting at that he came to earth untold years ago but chose for whatever reason not to wipe us out, perhaps to "train us through pain" and use us against the witness. Or they could go the "was humanity's originator narrative" . This would explain why the Pyramid ship on the moon is so old and why his glaive was there. Did we ever find out who the Salvation guy/girl was in beyond light? would be easy to say that was him.