I don't see anything from the link. Could you provide an actual quote from it? Also, those figures are very high. I can pull several sources where the numbers are a lot lower.
The woman that died from it only had 6 liters in 3 hours. Mind you, she didn't get a chance to urinate, so that of course worsens it.
Here is a 2001 article about soldiers and overhydration.
"soldiers drank excess amounts of water before developing symptoms and as part of field treatment. The authors conclude that hyponatremia resulted from too aggressive fluid replacement practices for soldiers in training status. The fluid replacement policy was revised with consideration given to both climatic heat stress and physical activity levels"
These soliders were monitored for a month and were healthy, no kidney problems. They were excessively drinking water for a while before symptoms appeared.
Also, you forgot to consider people with kidney problems, people on medications that enable water retention, as well as size and activity level of a person.
You also didn't take into consideration that people who drink a lot of water may develop an addiction to it and may develop hyperhidrosis. Sweating as you know, results in sodium and other electrolytes leaving the body. So even if the kidneys regulate everything, the sweating will still remove a lot of sodium.
You said long-term hyponatremia is not possible, yet it is. It could especially happen in infants, children, and people with kidney problems.
I'm not saying that you're wrong by saying the kidney can excrete 20l of water. "Thus, the maximum amount of water that a person with normal renal function can drink is 800-1,000 mL/hr to avoid hyponatremia symptoms"
But, I'm saying you can't say it's an impossibility. There is a small chance people can develop it and I would rather people not try and avoid other disorders and issues versus think it's okay to drink 6-12 liters of water a day. The body does not need that much water and it will act up because of it. Overhydration, a step before water intoxication, still produces harsh symptoms, even long-term.
"Symptomatic hyponatremia can occur when one drinks 3-4 L of water"
I also think it would be common sense to not make your kidneys go through that much effort each day.
*You didn't post this in r/medicine. Why have you not?
Source for the quotes: http://synapse.koreamed.org/DOIx.php?id=10.6065/apem.2013.18.2.95