Article 11 [Mexican Constitution]

  Every person has the right to enter and leave the country, to travel through its territory and to move house without the necessity of a letter of safe passage, passport, safe-conduct or any other similar requirement. In the event of criminal or civil liability, the exercise of this right shall be subject to the judicial authority. Relating to limitations imposed by the laws on immigration and public health, or in respect to undesirable aliens residing in the country, the exercise of this right shall be subject to the administrative authority.

Every person has the right to enter the country without a passport. There are ways for the authorities to get around it and fuck with people found in the interior without it (says subject to administrative authority for immigration, but they're explicitly constitutionally barred from requiring anything like a passport) , but ultimately it's unconstitutional to make a law requiring it. This trumps the aspirational hearsay provided by the consulate and explains why none of the consulate advice is able to cite where this supposed "requirement" comes from. The consulate either was mistaken or wrote that because it will really suck to leave the country without it and they don't want to deal with the fallout.

https://www.constituteproject.org/constitution/Mexico_2015#s...