WE may not understand DNA, but a being who designed a solution involving DNA would certainly, as the creator, understand it inside and out.
Also, the Intelligent Designer didn't HAVE to use DNA, right? The ID has unlimited options at his disposal for how to design the human being. The ID didn't have to give bipeds a spine and knees designed for quadrupeds. The ID didn't have to make our primary light source cancer-causing. The ID didn't have to use DNA or use DNA that is susceptible to mutation, right?