But it isn't a case of, "Omigod, who would ever have imagined that life could ever use any chemicals but those we already know it uses! I'm stunned. Amazed! Flummoxed! Flabbergasted!"
I guess I view it through a different lens. Biologists have many speculations about many things. It is going from speculation to verification that is the exciting part. Speculating that bacteria could replace phosphate with arsenate in something as fundamental as ATP is one thing. Showing it is another. To use an analogy, people had long speculated that humans could build a self powered flying machine. This didn't take away from the achievements of the Wright Bros.
As for the astrobiology aspect of the study, it is one thing to say that life could replace phosphates with arsenates, and it is an entirely different thing to show that it can indeed do it.