Correct, it is not category "theory". The terminology "category theory" here is used analogously to how database people say relational algebra is based on "set theory". It sounds a bit silly to mathematical ears.
On the other hand, the article is about much more than composing functions. In fact the section headings are "The function category", "The Kleisli category", and "The Pipe category". The rudimentary structure of a category is useful for putting disparate concepts on common ground.
Correct, it is not category "theory". The terminology "category theory" here is used analogously to how database people say relational algebra is based on "set theory". It sounds a bit silly to mathematical ears.
On the other hand, the article is about much more than composing functions. In fact the section headings are "The function category", "The Kleisli category", and "The Pipe category". The rudimentary structure of a category is useful for putting disparate concepts on common ground.