The U.S. was attacked. It invoked article 5. I don't know about you but I don't know how article 5 determines what is considered a 'valid' attack but I don't imagine a particular country's foreign policy over half a century weights that much. It's the same foreign policy that existed when NATO was formed.
Why did Canada and many other countries respond?
I assume you are aware of the reasons given for Canadian participation and how you ended up in Kandahar.