One factor of production

If you use only one factor of production, by definition, you can only produce a perfect substitute of that factor of production.

When, for instance, Krugman et al, in their book International Economics 9/E, page 26 et seq. speak about a basic model with "only one factor of production", they are not using the concept of "factor of production" as used in the economic theory of production. Not at all! They are merely using the term "labor" as a numeraire of unit of account.