Nobody doubted or denied that Zetas began as deserting Mexican special forces trained by the U.S.
It appears to me the "misleading" remark was in reference to the suggestion that most of the violent acts occurring these days are by those members or that the Zetas as a whole are this group of highly trained special forces.
I'll reiterate Neil's comment. "Of the ones who were actually trained in the US, how many are still alive today and what percentage of the total Zeta membership do they comprise?"
It would appear from their effectiveness in rapidly becoming one of the two most powerful cartels in Mexico that the original leaders are still largely in place or were very effective in passing on their knowledge and skills to their successors. In addition, the Zetas have expanded to Guatemala and have been recruiting former Guatemalan special forces troops, known as Kaibiles, who were responsible for some of the worst atrocities in the Guatemala Civil War of the 1970s-90s. One of the Kaibiles favorite tactics in Guatemala was beheading people to spread terror and there is evidence that Zetas may have picked up this idea from them.The Kaibiles were also connected to the US Army School of the Americas. One of their most notorious officers, Pedro Pimentel, was invited to be an instructor there in the 1980s, after he had committed some of his most notorious acts. US taxpayer dollars at work.