This is a real thing, and yes, people have gotten sick from the water on an airplane, it just doesn't make the news. Just like restaurants, schools, hospitals, etc., the water and food on an airplane is something that is supposed to be safe and meet standards, whether you choose to drink it or not. Airlines are responsible for cleaning their storage tanks. Water can come from any airport around the world that they top up at. An airline that has hundreds of aircraft and has to schedule and perform regular maintenance, can have problems on any single one of them, that won't show up without regular testing.
It's not the most important thing to worry about when flying, just something to know. I don't avoid it, and I know the risks.