Mugi/awa/hie were untracked substitutes for rice. Medieval Japanese warlords mainly collected taxes in form of bags of rice, and ignored other crops. So peasants mixed those grains into rice at varying ratios of up to 100% depending on local and yearly yields. That doesn't mean those grains were culturally considered defaults.

Your pdf is mostly about the 1870s and later. (Although it does mention the Tokugawa period, which began in 1603, it seems to do so for quite tangential reason.)

In contrast, people in East Asia started cultivating rice 9,000 years ago, and modern Japanese are probably mostly descended from these early rice farmers (who started out in China, then spread to Korea and then Japan) with a substantial contribution from another population called Jomon, which were already in Japan when the heavily-rice-dependent people started to arrive in Japan about 2,300 years ago and who lived mostly by hunting and gathering.

This is relevant because some people here are advocating for everyone to adopt a Japanese-like diet, which might not turn out so well for you unless most your ancestors have 8,000 or 9,000 years of experience getting most of their calories from grains.