That's fair, they didn't prove an "arsenic backbone," but here's a couple of key lines...

"...the purified DNA extracted from +As/-P cells contained As. Our NanoSIMS analyses, combined with the evidence for intracellular arsenic by ICP-MS and our radiolabeled AsO4- experiments demonstrated that intracellular AsO4 was incorporated into key biomolecules, specifically DNA."

and

"...the level of PO4 impurities in the medium was insufficient to elicit growth in the control (-As/-P)"