That map is just for Valero. Other oil companies and refineries are also shipping to Mexico from Gulf Coast Refineries. There are also refineries in California that may be shipping to western Mexico but I do not know. Once in Mexico they must still use Pemex infrastructure at this point - meaning storage tanks and transportation.
I think one of my previous posts referenced a website that said OXXO was going to switch to showing they have their own branded fuel - so OXXO could have either Pemex or some fuel from US refineries. It would appear that some Pemex stations are getting their fuel from the US - which ones could be hard to say. It would also appear that the Gulf branded stations will only be in the Mexico City area for the foreseeable future.
As Navigator said above - newer municipal buses in the Mexico City area are supposed to be using ULSD so there could be sources of ULSD around Mexico City. Mexico is desperate to clean up Mexico City air and having cleaner fuel is one important step. Could be Gulf Coast fuel to Vera Cruz is then transported to Mexico City. Speculation. Facts could be out there.
One of my other posts showing a website indicated that electric generating plants are switching from diesel or coal to natural gas due to the low cost of natural gas - and it is much cleaner fuel. I think that is the reason new natural gas pipelines are being built in the US. That would reduce the need for diesel in Mexican power plants. Time will tell.