Let's break down the actual meaning of the treaty, cutting out the extra stuff.
"... each of them will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area."
So, if the US deems no action is necessary on their part, because a bunch of other nations have it locked up, then they don't necessarily need to help.