All he has to do is direct the ABC boys to stop running guns to Mexico to fund their black budgets and that would cut down on guns going there, if they comply. Add to that securing the border, the flow of guns going south would be reduced in very meaningful ways.

Not that the cartels really need US guns, the black market is flush with firearms.

My guess is Trump believed he could deliver with just securing the border and has no idea about the government gun runners. He probably threw that out there as a freebie to add leverage, because securing the border would have the downstream effect of less gun running with zero extra effort.