Hasn't nearly all of society and humanity been wiped out thousands of times by this point? Like, Mitama was going on about how difficult it was to find a time where gods were totally completely unknown, wasn't she? It seems unlikely that the concept of "Doujinshi" would be something that wasn't totally wiped out and buried in the ashes of these thousands of civilizations. (And it's not a very easy to come up with concept, considering the fact that it pretty much only exists in Japan, even today!) So: either the author did an oopsie and forgot what their setting was or this was intentional and meant to imply that we're not as far in the future as Mitama implied. (Maybe she's just really lazy and even sifting through the twenty years it took her to get here was just too much for her... I guess it'd be in character.)