It should be pointed out here that biological genuses, families, orders, and so on do not exist. If you discover a new continent full of organisms not previously known, there is no observation you can make to decide whether two of their species are, or are not, members of the same genus. It would be a wrong question. Every classification above the species level exists solely for the convenience of biologists talking about the organisms they are studying. Even at the species level, where we can talk about interbreeding populations, multiple definitions are possible and edge cases exist (sometimes so large as to make the very idea of a tree of descent moot).

The higher-level classifications may (but do not always) correspond to subtrees of the evolutionary history, but their ranking into genuses, orders, subfamilies, and so on in the 40-odd different levels available in current taxonomical practice is a product of human convenience, nothing more.

So the statement that some genus includes only one species is not a statement of biology. It is a statement about biologists.