Based on the graph the process is producing way less CO2 than other additive processes. Being low temperature this intuitively seems a credible claim. Maybe you are concerned about high currents which is true but since voltage is low that does not multiply to much.
It's because that chart is measuring end-to-end energy use. Other metal printing approaches require a lot of energy to make the metal powders, so when you include that the other approaches are a lot worse.
it's an interesting point; you do of course have to add those electrons to the metal atoms in the first place when you're smelting it from ore, unless you're working from a rare native metal deposit, and plausibly you could leach metal ions out of ore and feed them into your 3-d printer. i suspect that the tests they've done so far, however, are using reagent-grade metal salts from sigma-aldrich or similar with much more embodied energy than metallurgical-grade copper or whatever
