If you look at the routes the Mexican buses are apparently allowed to take passengers between NY City and Miami, and between Chicago and NY City and it seems like LA also. Our government first made it sound like the buses and trucks would be allowed to travel from Mexico cities directly to American cities and wouldn't travel from one American point to another ---but this doesn't appear to be true ---even this early into the game.
NAFTA deals only with scheduled bus service. Mexican tour and charter service was, is, and will be available. You can assume there there is some rule bending going on.
The original date for scheduled bus service to commence for Mexican and US carriers was 1997, but of course it was delayed. Canada and the US had an agreement that predated NAFTA.
The DallasNews article from yesterday mentioning that Greyhound and Estrella Blanca had signed an agreement to interline their scheduled services suggests that neither plans to operate scheduled service cross border.