I don't recall this? As far as I could gather in the original DC One Million series, he was supposed to be Kal-El from the Post-Crisis Universe. But then, I would say that after the release of All-Star Superman, it actually makes a lot more sense if he's All-Star Superman - seeing as he is transformed into pure energy inside the sun, and then repairs it. It makes a lot of sense as the origin of Superman Prime.
Still, that would be really cool if he was Kal-L! That the first Superhero is also the ULTIMATE Superhero!
Not entirely sure if Superman Prime's character would be the way it is if he was Kal-L though... I always figured, he'd be a different type of Superman Prime... I mean, he IS a bit more set in his beliefs and a bit less diplomatic, really.