Why would you take this drug kingpin at his word? Let us leave aside the fact that we know right now that most of the illegal drugs our government has interdicted have come through
legal ports of entry (I would argue that this is because those legal point of entry are actually
guarded). But if you were the head of a major cartel with family members who run it alongside you, why would you give away all the ways and means your organization uses to smuggle drugs across the border into the United States? What is the incentive to tell the truth?
I have no doubt that El Chap knows that building stronger border security, including a border wall, is an issue of major contention within the United States and is dividing American society. If smuggling drugs across unguarded sections of the border is indeed one way he gets drugs into the United States, why on Earth would he give that away and say "Oh, yeah, a well-guarded border wall would probably hurt a lot of my drug trafficking operations."?