It says there that a codebase can contain multiple repos. They don’t mean a monorepo when they say codebase

> A codebase is any single repo (in a centralized revision control system like Subversion), or any set of repos who share a root commit (in a decentralized revision control system like Git).

They do mean that a repo = a codebase. A remark about "set of repos" is just a technicality of distributed VCSs, meaning that each copy cloned from a single source (or from other copies) is a single repo.