No, but he is and was the head of Al Queda. He made the declaration on the organization's behalf.
It's like saying Jack The Ripper has declared war on the United States.
Aside from the time frame issue, more or less. This is more like the Barbary Pirates ...And we'd be free to whack him in response.
We in fact are conducting a war against Al-Queda in addition to other affiliated entities.