Before the proliferation of wireless phones (even in Haiti!) there was the telebouche...word of mouth person to person usually initiated by radio.
I expect the Mexican gov paid Haitians or Creole-speaking Mexicans to spread the word that SleepyJoe had thrown the doors open and this was going to be a limited opportunity.
Add free bus rides to the Rio Grande area during this recent Mexican fete and voila!
In this human haystack; can we find the terrorist needles?