I don’t think it’s about japanese cuisine in specific. Medieval food before crusades and even after for the poor was probably pretty Bland. Even japanese, ancient rice was probably horrible, and tools and refining ingredients was way worse. If you introduce seasoning and ingredients processing that were developed over the ages i’d expect it to make food taste better. The japanese characters in the japanese written media make japanese food, I wouldn’t call that unexpected.
Also, why are you getting annoyed at a supposedly unexpected reaction from fictional characters in a fantasy fictional world? There’s so many more logical loopholes and convenient plot devices used by most isekai that i don’t think the medieval inspired world liking seasoned food is the worst there is to criticize. The food being awesome for everyone is the least of the problems, being able to reproduce our modern cuisine in a medieval world is the real problem
1. It's in the main plot, of course reader need to take a concern to it
2. Medieval people is not a complete moron. The ingredients is indeed can be scarce because technological issue
3. It's in no way make it more inferior. If you want to get a real research, go to your most secluded area in your country. If your country don't have it, go to country that have a tribe that deny technology implementation. They have methods to acquire the most tastier and highest quality that can't be produced by technology. For my example in my country, it's honey from the cliff's mountain area
4. Technology is just a tool. In modern area, it usually limiting the variations of ingredient so you would have less chance to find something that only exist in medieval era because of the owner of that business doesn't like it
5. Realistically, Japan's taste is based on maritime product because they are living near an ocean. The people in middle of continent would get a different preferences however good the food actually is. It's just like macD's vendor have different menus and ingredients in each country to match the local's tastes
6. I disagree with your last statement not because it's not true. It's because you shouldn't take an issue about that if it's not the main issue that the story talking about. Fiction is always weird. But you still can infer whether the author is a moron based on that