The US Mexican border is very porous. It is very easy to bring things into Mexico that you are not supposed to.
Pre Marxist president Calles in the 1920s everyone in Northern Mexico had guns, a hunting rifle, a shotgun and a hand gun. Part of Marxist president Calles genocide was to kill those who would not turn in their weapons for the good of the revolution. To not turn in your weapons was proof that you were part of La Cristiada.
Post Calles there is a long history of gun lovers bringing guns into Mexico from the US.
It was not that hard for the cartel, with money, to gear up.
Guess I should have added the /s or the disclaimer that the border was the south fence on the ranch. Zillion footprints every night on the dirt farm road .