Yea idk, I think that version is an attempt at a (pathetic) post hoc justification for the crime (not saying it was made up by gp, but rather the cartels or their apologists). I had read that they were public municipal buses, which I guess could conceivably be used for smuggling, but would seem like an odd choice, as it doesn't seem like they would have much need to "smuggle" within the the municipality where they already operate with impunity, and driving the buses out of the territory would seem impractical/surreptitious.