How do you know "empirically" that they aren't? Who says that the US-sourced guns that they are tracing are even a substantial fraction of the overall guns in use? How can you prove empirically that the data provided by the notoriously-reliable and agenda-less Mexican government is accurate?

Mexico, 10 years ago: https://www.vice.com/en/article/the-cartel-gunsmiths/

Philippines, 13 years ago: https://www.youtube.com/watch?v=67PYuGQM9Fg

I think it's still a relevant point. The point isn't necessarily that it's easier for cartels to make it themselves than to smuggle guns or divert them from military sources. It's that the cartels can easily replace smuggled guns with manufactured guns and their demand for them is inelastic enough at either price point it's unlikely to effect the access to cartels.

The more likely effect is it disproportionately stops normal Mexico citizens from obtaining "illegal" guns to protect themselves but the cartels still have them, making things even worse for the Mexican people.