perhaps they manufacture C in their livers as most all other animals do... or perhaps they eat the organ meats such as the liver and kidneys in those animals that manufacture C in their livers or kidneys...
To add a bit here not all meat and dairy is void of vitamin C...
The glucose needed to produce ascorbate in the liver (in mammals and perching birds) is extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process.[21] In reptiles and birds the biosynthesis is carried out in the kidneys.