I'm sorry but this is more political talking points than reality... depending on jurisdiction, many of these ag positions are exempt from minimum wage, where people are still paid based on production. That is - the wage is kept low by regulation, put in place by the people who benefit from keeping those wages low.
You aren't wrong when you say there are migrant workers, but it's important to put that in context. In commercial ag, there are many types of workers who migrate around the country (and yes, also into Mexico) following the seasons. So for example if you are a skilled asparagus picker (yes this is a thing, and not a surprise it is mentioned in the article), you may start the season in Mexico, work through CA, and then continue northward following the season. So it isn't just a matter of "wages no local person would accept" but also a matter of training and skill. As an employer, you want someone who has the training and skills who you can trust year after year. Predictability. You don't get that hiring the local high school kids for the summer. Yes, I know there are places like Iowa where corn detasseling is a right of passage, but that's just not the case for the bulk of west coast commercial agriculture.