One of the problems of text books is that by definition, they are used in the education of minds not fully formed. That means that regardless of how factual they are, presenting it the wrong way will lead to hasty conclusions and a failure to properly understand the implications of facts. I'm very much of the belief that we ought not represent fiction as fact, and due to the lesser mental capacity of the young and uneducated being taught, a higher standard is to be expected in the clarity of presentation of that which is fact and that which is fiction.
It would be all well and good if the book made it clear that the real reason that Mexicans were not preferred was because like today, people tended to be xenophobic and thus racist against Mexican culture and persons, and that this was not helped by the fact that SOME Mexicans unfortunately fit the stereotype and we're used to demean not just those people who failed to work well, but to demean everyone of that already-disliked group. If the point of history classes is to learn from our past mistakes, it is good to recognize that they were in fact mistakes within the text. The text presented, however, does not seem to recognize the mistake of employers to use population trends to evaluate individuals without respect to the actual individual.
Have you considered the possibility (without additional information) that the book is being overly generous to 19th - early 20th century Mexican culture and people in the region specified? What if the Mexican culture really WAS lazy and drunken in that area and a diligent, productive, Mexican Worker was hard to find? Can you say 100% certainty that "good" Mexican workers weren't hard to find in the specified region at the time? In this case, the authors would be actually probably be injecting a liberal bias by only reporting the opinions of the employers at the time and not the actual disappointing work tendencies prevalent throughout the Mexican Workforce.
There's a reason I mentioned that I need to see the sources to make a decision on this text book. Facts and context are important. If it's a fact and it's relevant to the subject it can be put in a text book. You don't have to explain the difference between a fact and an opinion every time you mention one or the other. You don't have to highlight and deconstruct every logical fallacy, absurdity or hypocrisy committed by people of the past either. As I understand it, this is a book focusing on Mexican culture and history, not logic, and ethics.
Also, sometimes you only report the facts that are available. If there is no good evidence pointing to the objective quality of Mexican employees but there is good evidence pointing to a consensus of opinion from employers, then can you guess which one is more likely to show up in a text book? When you don't know if the prejudices of the employers were justified or not, it's a bad idea to strike off on a tangent prematurely damning them or conversely absolving them.