Just curious, do you have a cite for this? I have heard this several times and it has always struck me as an urban myth.
I think their point is, there is no such thing as “new” water, it is always recycled.
I remember a College Professor saying that the likelihood that any one of us is breathing in a Molecule of Air that Hitler exhaled is very likely. As I recall, he wasn’t joking at the time.
We live in a closed loop, a Biodome of sorts. Nothing we have here is being added to from anywhere else.