I don't believe that the US has ever said that "a digital attack is an act of war". The quote that you linked to says that the United States reserves the right to respond militarily to "hostile acts in cyberspace" if it exhausts all other options and judges the costs of action to be greater than the costs of inaction.

The statement is not saying that any cyberattack is an act of war, it is saying that the United States might treat certain attacks as the cost of doing business but that other attacks might require a military response, depending on the specifics of the incident.