There already is more than DNA on the pizza. There was his DNA in other areas of the crime scene. There was blood from the murder scene found on his shoes. And he had many hundred dollar bills on him when he was arrested, which were consistent with the denomination of the ransom money. And he took a very expensive cab ride from DC to NYC on the night of the murders. And when he saw himself on the news, he told his gf and his family that he was turning himself in. And apparently he told his ex-attorney that he was there but he was not the only one there.
Just to keep things as kosher as possible, he took a bus from DC to NYC after hearing he was wanted for the crime, a few days after the murders. Took the Uber/whatever ride back from NYC to DC. That is all.