There's safe and safe. 20x the safe dose of cannabis will make you very stoned for a while, 20x the safe dose for alcohol will kill you. Similarly an overdose of vitamin D vs aspirin.

The relevant metric here is LD50 - but a quick google doesn't find a number for vitamin D. A wild guess would be that the patient in question could have died, though whether the odds were 50% we can't really say from just one case. But it means that 130000 UI daily for over a year is in the ballpark.

taking 20x the recommended dose of water (3-4 liters daily - 60-80 liters to compare) will kill you too.

...and people have died from it: https://en.wikipedia.org/wiki/Water_intoxication