I interpret as the latter:

> As a rule of thumb, one koku was considered a sufficient quantity of rice to feed one person for one year.

I assume "sufficient rice" means it needs to be supplemented, and this is supported by the footnote as well:

> Apparently 1.8 koku (1 koku and 8 to) was actually required for nourishment by a man each year, according to the conventional wisdom documented in a "home code" (kakun [ja]) of a certain merchant family in the Edo period.