NATO also doesn't specify that the attacker is excluded if they were a NATO member. It just states if an attacker hits one of the members article 5 goes into effect. So an attack on Canada brings 31 countries against the U.S.
Yeah, start a war with a nation you share the longest unsecured boarder with, filled with people who look and sound like you, who also have a history of record geneva convention additions. That sounds like a great idea, what with the USs great track record at winning wars of insurgencies.....