Urine contains salt, always, even when in a state of hyponatremia: https://www.sciencedirect.com/topics/medicine-and-dentistry/sodium-excretion (scroll down to the kidney disease paper, it wont show any of the text on the direct link, insert obligatory hate on academic publishers)
I hope you don’t need a source for distilled water not containing salt or water needing to be excreted or for sweat (the other way water leaves your body) containing salt, I already spent way too much time on this because sourcing on mobile is a pain.
And yes, <10mmol/l isn’t a lot. That’s <500mg (and how low it can go precisely idk, couldn’t find that, but likely much lower, given that the <10mmol figure is a threshold for diagnosis of kidney issues) You replenish that through food, easily (esp these days where sodium intake is, if anything, very high). That’s the whole point. Barring very extreme situations, healthy kidneys will regulate your sodium levels just fine.
It’ll never stop being funny to me how in the US it’a apparently considered pretty bad and even gets censored while here in germany it’s one of those child friendly swears that you expect kindergarten kids to say.