We correct an overestimation of the production rate of 137 Xe in the DARWIN detector operated at LNGS. This formerly dominant intrinsic background source is now at a level similar to the irreducible background from solar 8 B neutrinos, thus unproblematic at the LNGS depth. The projected half-life sensitivity for the neutrinoless double beta decay ( 0 ν β β ) of 136 Xe improves by 22 % compared to the previously reported number and is now T 1 / 2 0 ν = 3.0 × 10 27 yr (90% C.L.) after 10 years of DARWIN operation. [ABSTRACT FROM AUTHOR]