In fact, there is a (rather uncommon) version of "people" which is singular, and that is what would be interpreted in those sentences. It means something totally different. The singular word, "people" (plural: "peoples"), means, as Merriam-Webster puts it: "a body of persons that are united by a common culture, tradition, or sense of kinship, that typically have common language, institutions, and beliefs, and that often constitute a politically organized group". So you could say that e.g. Okinawans are a Japanese people, Michiganders are an American people, and the Cherokees are an American Indian people.
I don't know why, but I tend to prefer "Japanese person" over the noun form "Japanese". Both are correct, though.