If it is true that Vitamin C breaks down to Hydrogen Peroxide, which in turn kills certain cells in the body, it IS feasible that Vitamin C could help treat COVID-19, and increase recovery times.
As stated in the post above, this happens when the liver breaks it down, so it doesn't generate the hydrogen peroxide in the body.
The problem with "X kills Y in a test tube" thinking is our bodies are not test tubes. Know what kills viruses? Napalm. Pour some napalm in a petri dish and it will kill every virus in it.
But nobody is saying, "Hey... maybe we should inhale burning napalm! It will kill the virus in our lungs!"
Sure, and then kill your lungs, and you. Hydrogen peroxide kills anything it touches. It doesn't JUST kill infected cells. You can't drink it or inject it safely.
Vitamin C isn't magic. It's just a chemical compound, one of thousands our bodies use. There is zero evidence to suspect that is specifically has some kind of super effect against COVID or other viruses.
We all want a magic COVID cure but Vitamin C just isn't it. We're just going to have to wait for actual researchers to find it for us.