For what it’s worth, I did a google map view of the border area of Clint, Texas where the story says this happened. I don’t know how authoritative Google Maps is, but it shows places where the actual border does not run down the middle of the Rio Grande, but some yards north of the river. This could be due to the fact that rivers do sometimes change their courses over the years and it has been agreed that the border will not change when this happens. Or it could be that Google maps is wrong. But it may not be as clear cut as we might think.
Google maps is incorrect. The southern border is defined by the center of the Rio Grande river. The river does alter its course, bit what gives us land in one area will lose it in another.