Wouldn't a jw just rationalise this somehow as the gene was broken when eve and Adam sinned and it's part of our inherited imperfection?
That wouldn't work as the same gene is broken in exactly the same way in our nearest evolutionary relatives. Clearly the mutation happened in a common ancestor of humans and other primates. Look at the picture in the OP.
Imagine you are a teacher correcting the homework of a group of students. If they all wrote that the Battle of Hastings happened in 1066 that would only suggest they all properly read the same textbook. There is only one way to get the answer right but there are an infinite number of ways to get it wrong. If 3 pupils all said that the answer was 1977 then you would have very strong evidence of copying. If you found many such identical errors in the same 3 papers you have proof.
The broken vitamin C gene is that sort of evidence.