“will assist such Party or Parties” comes right before that, though. Supporting an attacked treaty member is not optional.
And the clause which follows your quotes takes as granted that action has occurred, since it specifically states an intended result is a return to stability in North America or Europe. The action it deems necessary is predicated on the fact that it’s responding.
The way you are interpreting this quote is taken out of its context, which is not how the law works.
In any case, both of these arguments are technically valid, and it comes down to a whole lot of other factors, including political will, to enforce a response among members.
However it’s not ambiguous that an attack on a member of NATO will have a joint response, and a member neglecting to undertake such action would not have a valid legal argument for its inaction.
Edit: made a sentence real English instead of gibberish.