Is this a major concern? The last time I worked in Mexico I noticed a number of bottling plants turning out a variety of softdrinks, none of them had caravans of trucks loaded down with sugar or anything else made in the U.S. waiting to be off loaded. Mexico on the other hand,
exports quite a bit of fruit, vegetables and other items to the U.S. it would seem counter productive for them to begin a trade war in which they stand to lose more than they could possibly gain.
I did a rather quick search to get some background on this and found nothing more recent than 1996, in that year the trade between Mexico and the U.S. in sugar was nearly equal (imports and exports were about 27,000 metric tons) this down from an imbalance in 1990 of nearly 235,000:92 (metric tons)
http://www.fas.usda.gov/itp/policy/nafta/sugar.html
Where is your source for this article? I would be interested in learning more about what is going on.