Perhaps the title it trying to highlight that a single state of one country accounts for more trade than an entire continent and a multitude of countries that when combined still only make up Mexico second biggest trade partner?

They could have done the US in its entirety, but then it would be nearly ~$800b and would dwarf every other trade partner; or they could have compared it to all of North America, which isn’t that much different from just using all of the US—but when you look at just Texas, its still the single largest trade partner, ahead of the entire continent of Asia, but its a bit closer and more comparable while still highlighting the extreme difference in trade volume.