AU explained!:
(Manga spoilers if you haven’t read the manga)
As you know Muichiro dies in chapter 209. Years later after the battle with the Demon king, Tanjiro dies due to his Demon slayer mark. Muichiro and Tanjiro get reborn, but into a different universe where the timeline is different. Goes back hundredth years ago. Muichiro gets reborn way later after tanjiro did.
The rest goes as same.