My own theory regarding Dredd and SD is that the war that created Johnny, and which is concealed in the Alpha Files, was between Brit-Cit and Sino City One. But since, Sino City One is supposed to have been destroyed in Judgement Day after being overrun by Sabbat's* zombies, this cannot obviously happen anymore so the timelines have diverged**.
*so basically my entire theory boils down to "A Wizard Done It"
** Unless the destruction of Sino City One has been retconned/ignored/forgotten (delete as applicable), in which case - Sod it your guess is as good as mine.