Even the article says Mexico purchases large amounts of agricultural products, so why is this a contradiction?
Are you believing Mexico will no longer purchase these products?
The article says "Mexico" purchases large amounts of agricultural products, by which they almost certainly mean Mexican citizens and businesses, not the Mexican government.
Trump's tweet says that "Mexico" agreed to
begin to purchase lots of ag products. He could mean the Mex gov't agreed to buy them, which doesn't make a lot of sense. He could mean the Mexican gov't agreed that their citizens would begin to buy more, which also makes no sense (what gov't could possibly think it could get away with forcing it's citizens to buy things???). Therefore, I would conclude that, intentional or not, his tweet was off the mark.