> Turns out Mexico doesn't legally require a passport for entry
This isn't true. See e.g. https://consulmex.sre.gob.mx/washington/index.php/ligavisos/... :
> "All foreigners, regardless of their nationality, are required to present a valid and not expired passport or travel document when entering Mexico (traveling by air, land or sea)."
What you may have observed is Mexican border control at a small crossing may not enforce that requirement.
https://www.constituteproject.org/constitution/Mexico_2015#s...