True, though CDMX seems to be used more commonly than D.F. these days.

I've changed the title for clarity, though of course non-Duty Free alcohol is regulated by the same import rules as Duty Free. /Moderator

"If you are bringing alcohol into Mexico from the U.S., you are allowed to bring up to three liters of liquor or beer and six liters of wine per adult. ... Passengers over 18 years of age are allowed: 10 packs of cigarettes, 25 cigars or 200 grams of tobacco, three liters of liquor or beer, six liters of wine."

The original duty status of the liquor, wine or beer is unimportant. It only means duty was waived by the country of purchase, not ensuing countries. E.g. you might buy a single malt Scotch at LHR or fine Tequila at MEX duty free, but US duty may still apply on arrival at any other country.

Now, as to "tainted liquor" in Mexico as a reason: I've seen not one iota of evidence or allegation any bottled alcoholic beverage in Mexico is "tainted". There's greater risk in the US of that in cities where crooked barkeeps sell empty bottles of upper shelf booze to refillers or refill with their own home blends to make a bigger profit from selling fake booze.

There have been allegations of tainted alcoholic drinks served at some resorts. Personally, I pretty much dismiss these allegations as statements made by people who have overimbibed mass mixed or straight drinks made with lower shelf cheap booze. I've witnessed this many times. They're not using the finest "reposado" tequila at Carlos and Charlie's when they pass by with the bota to pour hooch down your gullet, and too much will make you sick, hung over or in some instances could kill of acute alcohol intoxication.