It might be plausible if any evidence was presented, otherwise it's very loose speculation.

Many of the cowhands working side by side vaqueros were literal boys (15-17) and very young men.

In Texas it's very common to call a young man a boy.

Currently it's a baseless accusation that it was a derogatory term, even moreso that it's racial.

It's never been mentioned as such until now, well past the high days of cowboys.