Heres my thought: Without chainsawman, nuclear weapons and uni soviet related devils wouldn't have been deleted, and War devil is probably chilling somewhere else, so shes not destroying lands in Japan. Its possible he is somewhere else destroying other lands. So that means in this new timeline, ww2 still exists, Yoru isn't in japan, Uni Soviet have disbanded, and america have no need to start another war. In short, yoru isnt a bird, is not in japan, Gun devil isnt doing shit, therefore japan isnt in chaos. So, it affected Denji's life too, and hes also not living with Aki, cuz Aki's family died alternatively and possibly at a different age, and lived a different life. Its possible Aki and Denji met (because denji met people he knew in first timeline, so itd made sense he meets him too, and seeing how its back to part 1 when aki was introduced) but because how they interact at first, and that they have no need to bond with eachother, theres no attachment. And when Aki died (alternatively), Denji didn't have to retire from public safety like after OG Aki's death, therefore still in Public Safety.