Agreed, the knowledge of avoiding scurvey goes back to at least the 1850s. Though the mechanisms of why it works are not known concretely.

If someone is concerned about Vitamin C, I don't see any problem with them taking a vitamin supplement - it's just unnecessary.

However, I don't think I've ever seen anyone who both wanted to try carnivore and was concerned with the vitamin c difference; Usually this dietary difference is used as a foil to fight against any research into carnivore (it's too dangerous to do a RCT, etc).