That exactly right, high dose vitamin C that doesn't go through the GI tract (given IV) is pro-oxidant rather than anti-oxidant.

Intravenous High-Dose Vitamin C in Cancer Therapy https://www.cancer.gov/research/key-initiatives/ras/ras-cent...

High-dose vitamin C enhances cancer immunotherapy https://www.science.org/doi/10.1126/scitranslmed.aay8707