Zenitsu wrote a journal about his life experience but modern kids don't believe in demon anymore, so touko (the girl who looks like nezuko) refers to the journal as 'novel of lies'... Yoshiteru (black-haired zenitsu in this chapter) is great grandchild of zenitsu
So youre telling me all of that were just ZENITSU MANGA?! AND THAT HE WAS IRL ALL THIS TIME