That is describing the profession, the article is describing the origin of the term used to describe the profession. A “vaquero” is what we now would call “cowboy”, and we call them “cowboys” because it was originally applied to Black slaves who worked with cattle.

There’s no way to say the word “cowboy” originated from the word “vaquero”, because there is no similarity phonetically, nor does “vaquero” specifically include the Spanish word “boy”. (un cocinero is a “cook”, not a “cookboy”.)