This is a GOOD thing. Easiest route to Texas from Chiapas is through Veracruz's coast up through Tamaulipas. It's the main truck route in Mexico.
The fact that they want to bus them into Mexico City is HUGE, it's going to make the walk far more difficult and they might even just disappear in Mexico City.
Mexico City is @12,890 ft elevation. Veracruz is basically at sea level.
They will make them walk up and down mountains to get out of MX city.