You're probably right, most things taken at 100x the normal dose for long period of time are toxic. (Water included)
While possible, it does seem unlikely.
Taking 100000 IU per day for months/years is not necessarily toxic, especially when reducing calcium intake.
I'm curious why an acute high dose would be preferable over a lower dose taken daily.
It's not quite an acute high dose. The body produces the equivalent of 10,000 IU per day in high sun exposure, so 60K per week tries to achieve that level. I believe the high dosage is simply convenience. The 2K IU daily dose is pretty common as well.
[1] - https://www.google.com/search?hl=en&source=hp&biw=&bih=&q=si...
(I have already been taking all three as supplements for the last year due to other research suggesting that having a shortage of them worsens ADHD symptoms)
Taking vitamin D for the average American will surely help in keeping good immunity, bone health, etc.
https://www.acc.org/latest-in-cardiology/clinical-trials/201...
I do wonder though if this is an important caveat though:
> among healthy participants.
.. since the rest of the discussion resolves around adding supplements to fight shortages. If "healthy" implies not have said vitamin D shortage then that is still a useful study but doesn't disprove the thesis that taking supplements is probably a good idea.