The advantage they cite is protection from seasonal scarcity due to needing less vitamin C. So during resource deprivation there is a window where the mutant only suffers from conventional malnutrition, whereas the non-mutant's mechanisms for creating endogenous vitamin C are also impacted. So the non-mutant suffers from both malnutrition AND scurvy, which presumably kills them faster. Eventually, the mutant would also suffer from scurvy.

That's mostly speculative though, there would need to be some kind of starvation study done between species to figure out which one dies first. That would be unethical :(

Losing endogenous production of vitamin C was not a strictly negative mutation as you're making it out to be.

https://academic.oup.com/emph/article/2019/1/221/5556105

This paper, for example, outlines that the total amount of required vitamin C is much lower, because scavenging and recycling machinery related to electron transport became more efficient as a result of the mutation.

More simply, it's like the difference between cooking for yourself and ordering out. Does cooking for yourself make you more independent? In certain contexts, yes. Is ordering out VASTLY easier and more expedient if you don't care about the money? Sure. And then you can use the saved time to do other things you'd like to do.

Is one better than the other? Depends on context. In our context, it's generally pretty easy to hit vitamin c requirements: you spend like a minute eating an orange and you're good. If you're on a boat for months and months without access to vitamin C, then that context will suck to rely on exogenous vitamin C.