Interestingly enough - the US is the only country to have invoked article 5 which is the principle that an attack on one member is considered an attack on all - in response to the 9/11 attacks.
To nitpick: it was not the US that invoked article 5 after 9/11. It was driven by its European allies, which wanted a show of support at that time, to be expressed any way they could.
The actual impact of that article 5 was pretty non-existent. 9/11 was not a declaration of war by a state; it was an action of a well-organized group. There are no specific actions that this article 5 could imply beyond the PR.
I do not think you are wrong. It was a joint declaration, but from what I vaguely remember, the idea came from Europe and everyone understood it to be a great PR value with no technical pull. My 2c.
The actual impact of that article 5 was pretty non-existent. 9/11 was not a declaration of war by a state; it was an action of a well-organized group. There are no specific actions that this article 5 could imply beyond the PR.