

60 was chosen by the Ancient Sumerians specifically because of its divisibility by 2, 3, 4, and 5. Today, 60 is considered a superior highly composite number but that bit of theory wouldn’t have been as important to the Sumerians and Babylonians as the simple ability to divide 60 by many commonly used factors (2, 3, 4, 5, 6, 10, 12, 15) without any remainders or fractions to worry about.
Lots of people drink bottled water, soda, beer, or other drinks not immediately connected to the water supply. Furthermore, poisons are unlikely to remain undetected long enough to kill the entire population. While a strong dose of a deadly poison like cyanide can kill in minutes it’s likely to be detected quickly due to how rapidly its effects begin to show up.
A slower-acting, accumulating poison like dimethylmercury could potentially kill more people because its effects don’t show up immediately. On the other hand, the delayed effects of the poison would provide the victims a chance to retaliate against the poisoners.
Either way, it’s a very crude and unfocused attack against a population which is unlikely to achieve any political aim besides wanton destruction and outrage.