It is not an isekai. Isekai means another world that is different from our world ie. different animals and plant species, the introduction of magic, etc.
If time travel is isekai too then Ouke no monshou, Red River, Amakusa 1637, Jin, Nobunaga no Chef, etc. are all isekai?
People should go look up the very definition of isekai and stop abusing the word.
@VerneJules She just said it that way for Nobunaga to understand easier. And you should remember that Japanese is complicated. What we read is just the translated version so it might not be the same words the author originally wanted to convey. One example, there are a lot of works translated the word province and domain as nation (ie. the official translated Inuyasha), leading people thinking that Japan was split into many nations in Sengoku era but it was not the case since all the feudal lords still had to obey the Shogun and the Emperor.
@crazybars Rich people in ancient Japan didn't eat meat regularly either since Emperor banned meat eating since Heian period. The main source of protein for them were fishes and other seafood. Sometimes they ate boars and ducks but beef was a big no. Chicken were actually raised as pets. Only mountain people and hunters ate meat frequently but they were usually shunned by other people and labeled as barbarians. Remember in some of the first chapters when Shizuko hunted the deer, the villagers were afraid of eating it.