This reminds me of a famous rver from a few years back that swore up and down at the time that ATMs didn't exist in Mexico although the person's experience was quite limited and traveled the most barren parts of western Mexico and just didn't know better. Vast generalizations.
I don't care for these threads as it spreads some old myths as well as misinformation about Mexico and scares off possible Mexophiles.
That said, the vast majority of Mexicans (77% live in urban areas) shop in supermarkets and are always careful about what they buy and eat.
However many Mexicans and tourists eat on the streets daily using common sense rules.
We may be behind in some things but we're definitely not living in the 1800s either.