Honestly I've read enough Isekai where I simply don't believe that previous reincarnators didn't already introduce cooking and ingredients from Japan
It's probably my least favorite aspect of Isekai outside of Idiot Harems: Depicting highly developed setting as being ass-backwards to the point of not knowing how to make bread and how fan-fucking-tastic Japanese writers try to make out Japanese cooking to be despite it being characteristically and historically bland as fuck and benefitted the most from foreign idea exchange.
Y'all were eating brown rice, millet and mountain yams and until 1860, you didn't have bread until the Portuguese introduced it in the 1600's and you didn't have it as a staple until after WWII due to food shortages. Sit the fuck down
I like Japanese cooking, but I know it well enough to know that the most popular japanese foods are just permutations of European, Indian or Chinese cooking.