There’s only so much Vitamin C your body can absorb and any excess is excreted via the urinary tract, true. All these studies touting the benefits of very high Vitamin C dosage depend upon either injections or possibly sublingual liquids which would be absorbed under the tongue. “Bioavailability” is an issue with a lot of supplements, tough to get to the therapeutic dose orally.
If the person is deficient or if the body needs it at the time, is that when the body will keep it instead of pee it? I’m sure athletes eat differently off season (or should) than during season.