@CelticMutt : The same way they had corn but did not know about popcorn...(there is evidence of popcorn dating back over 5000 years)
Though this is one of the things that tends to make no sense about many isekai worlds, food items existing but not being taken advantage of and it is almost always food items that were not a part of the ( often highly incorrect) opinion of a medieval European diet (it was no where near as bland as many authors make it to be). But normally the foods are ones that were not native to Europe (potatoes originated in South America)
Though if you are not starved for resources, and have abundant food supplies, it can be excused to have not tested all the plants in the area, the real world had a lot of periods with scarcity of resources forcing the use of every resource. and this is a post collapse civilization that lost ALOT of knowledge.