When I read they went down 260 ft, I thought, "Dang, they were on mixed gas for a cave dive!" But, I think you're right, the reporter mixed up distance with depth.
After reading more about this cave complex, it’s apparently quite possible that they intentionally dived that deep, which I guess would mean they would be using mixed gas, as you say. Still seems nuts to me, why do people have to take already dangerous activities and push the danger right to the limit?