I consider the fact that humans and chimps share an **identical mutation** that prevents the synthesis of vitamin C as proof in and of itself. Don't you?
What do you consider whales that are occasionally born with legs? Evidence that [the designer] was so stupid [he she or it] put land animal genes in a marine animal, or evidence that the whales inherited these genes from a terrestial ancestor?
It's certainly an interesting piece of evidence for common descent. But calling it "proof" might be a bit of a stretch.
The evolutionists certainly do say the above. However, there is a problem with it. When we mapped the human genome, two companies did it. Only 1/5 of the genes they identified were the same. I am not sure that the chimp genome has been studied even better than man. So first of all, I am very suspicious of that statement. Secondly, that would be an example of devolution - making a species less fit. Mutations seem to do that. Thirdly, I do not know how large that gene is, but if is like most genes 500 or some base pairs long, that one mutation made it unworkable in both man and chimp is not to be wondered at. It is a slim chance, but not an impossible coincidence. Lastly, the genes of different species are never the same even if they code for the same function. That is why the sperm of one species will not impregnate another species, why the blood of one species cannot be used on another species, why the legs of one species are not the same as those of another species, etc.