@zetsuravez @Magneticmagnet and why not
@Siceraria and
@blablablub
I also agree on the oats and potatoes comments.
There are plenty of plants that are thought to not be food, or are used primarily for animals in certain places.
Soybeans - while many people now know of tofu and soybean oil, most of it is used for animal feed. https://www.canr.msu.edu/news/where_do_all_these_soybeans_go#:~:text=Corn%20and%20soybeans%20are%20both,%2C%20for%20fiber%2C%20and%20energy.
Sorghum - 1/3 of the crop in the US is used as animal feed, and the US is a top producer of the stuff, but it's not widely known in food. It's used elsewhere as food, however. I only came across it in regards to gluten free beer and learned that it could be popped after that by accident.
Quinoa - now a commonly known crop, there was a popularity boom through the early 2000's, prior to that, I'm not sure how well known it was as a food source beyond certain subcultures in the US.
So rice could be viewed the same way, where the culture doesn't see it as a human food source. A quick search shows that rice was consumed around the world. BC eras. And Europe in the 10th-15th century, so there's more history there than what's easily known.
Honestly though, it'd make more sense for Soybeans to be seen as an animal food crop and then the isekai'd mc searching for it to make tofu and other products and have rice be a food staple, even if not as common as wheat. Rice could be seen in a pilaf, or a risotto, or added to soup and not necessarily steamed too and wheat took over due to bread popularity, at least steaming would be slightly novel.