The study didn’t show anything to support the claim. From the very article you posted: “Although body fat mass decreased slightly more in the vitamin C group, approaching but not reaching statistical significance, both groups lost an average of nine pounds, indicating that vitamin C depletion did not appear to affect the ability to lose weight in the short term”.
And I suspect the study is just a poster presentation and not yet published.
It is basically called the hormesis concept.I used to work ina lab which studies the effect of antioxidants on muscle loss .
Too much of antioxidants can blunt the inflammatory pathways and decrease muscle growth and other beneficial adaptations. We know that free radicals are not evil and are signaling molecules and have a lot of useful functions. In your body there are endogenous antioxidants like glutathione, superoxide dismutase and so on. Their production of these are triggered by free radicals and other inflammatory substances. So if you suppress too much of these, you suppress the production of these endogenous antioxidants.