Maximum tolerable uptake for Vitamin C is 2 grams per day in adults. These guidelines suggest 8 grams per day prophylaxis and then something like 10 grams daily in Early Treatment. That seems crazy to me. I would definitely get diarrhea.
Yes it's important to keep in mind cost vs. benefit and prophylaxis vs. treatment. See this LitCovid study, vitamin C is being investigated as a treatment in moderate to severe cases.
But the megadosing movement is likely just more quacks seeking out an easy, at-home miracle cure. If you read the vitamin D studies on LitCovid, you'll see they all clearly involve "deficiency" in the title. They are not extolling the virtues of supraphysiologic dosing
Nevertheless it's interesting,
RESULTS: Regression modelling shows that more northerly countries in the Northern Hemisphere are currently (May 2020) showing relatively high COVID-19 mortality, with an estimated 4.4% increase in mortality for each 1 degree latitude north of 28 degrees North (P=0.031) after adjustment for age of population. This supports a role for ultraviolet B acting via vitamin D synthesis. Factors associated with worse COVID-19 prognosis include old age, ethnicity, male sex, obesity, diabetes and hypertension and these also associate with deficiency of vitamin D or its response. Vitamin D deficiency is also linked to severity of childhood respiratory illness. Experimentally, vitamin D increases the ratio of angiotensin converting enzyme 2 (ACE2) to ACE, thus increasing angiotensin II hydrolysis and reducing subsequent inflammatory cytokine response to pathogens and lung injury.