This is sort of how everything works, unfortunately! Guaranteeing 0% of something is really hard. Your flour probably has a small percentage of bugs in it, for instance. Urea is a relatively small molecule that I imagine you can find tiny amounts of pretty much anywhere. I would be unsurprised if there was at least one molecule of urea in literally anything you eat!
That said, dear god I hope I’ve never been in a pool that’s 10% urine :(. Those kiddie pools at the water park are probably like 90% urine, though. Sometimes I wonder if by volume adults pee in the pool more than kids, though. I have a suspicion a good chunk of adults think it’s fine or will do it secretly anyway.