To clear up a few of the other questions, they didn't measure where mama was getting her fluoride, or how much was in her tap water, or even how effective she was at excreting it. They also didn't measure postnatal fluoride exposure by the children, which may be less, more, or just as important.
Mama could have swallowed some toothpaste or mouthwash even though she lived on unfluoridated water. She could have eaten a steady diet of fluoride-infused potato chips manufactured in a place with fluoridated water even though her water was unfluoridated. Or, she could have been a tea drinker, which is a leading source of fluoride.
Absent evidence to the contrary, I believe this study's conclusions are the right ones, but the study could do little to back them up. For all we know (and for all the efforts the study didn't make to rule it out), higher fluoride exposure could simply be a reflection of poverty, which would similarly correlate to lower childhood IQ's.
If you drink untreated water in Mexico, you may very well be drinking fluoridated water, which occurs naturally in some spots. El Paso and Albuquerque have naturally fluoridated water.