This statement seems the closest for that conclusion: >Bedford’s analyses of RaTG13 and 2019-nCoV suggest that the two viruses shared a common ancestor 25 to 6

This statement seems the closest for that conclusion: >Bedford’s analyses of RaTG13 and 2019-nCoV suggest that the two viruses shared a common ancestor 25 to 65 years ago, an estimate he arrived at by combining the difference in nucleotides between the viruses with the presumed rates of mutation in other coronaviruses. So it likely took decades for RaTG13-like viruses to mutate into 2019-nCoV. But if the mutations are natural, that just means it wasn't engineered, it doesn't mean like you have stated, that it excludes storage in a lab at some point. Also not all activity happens in the lab, for all we know it could of been outside the lab and an affiliate exposed during capturing or during transport of bats to sample, breathing in virus particles exhaled or otherwise exuded by the bats, perhaps when their guard was down, not expecting to get sick.