Plenty of federal facilities have garbage reception. I think it’s probably due to the bureaucracy involved in telecoms installing their hardware on sensitive property. The White House in particular probably has lots of thick walls/armor attenuating signals, too.
Flint is a very small percentage of the population, so even if everyone in Flint was affected, which they weren’t, it would still be possible to get a 100/100 score. The problem with Flint’s water was highly publicized because of how uncommon it is for water to be unsafe in America.
The legal limit for lead in drinking water in the EU is 10 ug/L. Lots of places in the EU are above that.
Flint, at the height of the crisis, had a median of 3.5 ug/L, but 17% of samples were over 15. Compare that to the study I linked, which shows Vienna having 18% of samples over 25.