That’s the ending in my Absolute edition, but it could be a later addition to the story. Never read that JSA arc (or any of Johns JSA to be Frank) so I wouldn’t know.
Oh he’s definitely going off to space, but Superman & the Authority takes place on Earth, so it’s not inconceivable that while Clark will be primarily off-world and Jon will be primarily Earth based, both of them could operate on Earth and in space concurrently. We’ll see how they divy up the responsibilities is what I meant.