If this map is correct, it is possible the encounter was just north of Clint, where it appears the actual border for a bit is to the Mexican side of the Rio Grande—and so the US soldiers could have been on that side of the river, where the Mexican soldiers mistakenly indeed thought they were in Mexican territory.
But if Mexican soldiers are patrolling right up to the border, we need a greater show of US soldiers guarding on our side.
It wouldn’t be the first time that a meandering river has caused border problems.