Well, the term "buckaroo" is believed to be an anglicized version of "vaquero" showing phonological characteristics compatible with its Spanish origin. The word "buckaroo" first appeared in American English in 1827. It's still used to refer to a certain style of cowboys and horsemanship, particularly in the Great Basin region of the United States, which retains characteristics of the traditional vaquero.
But to your criticism, the transition from "vaquero" to "cowboy" just reflects the influence of Spanish culture on American ranching practices.
Vaqueros and cowboys worked side by side so it seems to be a natural transition between the two and has much more support historically and linguistically than the complete lack of evidence provided by the article.
Words don't appear out of nowhere, they naturally evolve and generally are based on imported words and root words, and it makes sense that Spain, the ones who created this practice and imported these cows and horses were the root of the evolution, beginning from the latin root word "vacca".