That's true, but I also think it's more of a formality than anything else -- since NATO has also been used in ways that have nothing to do with the mutual defense of its member nations. I've always viewed NATO as little more than a legal/diplomatic mechanism under which the U.S. has basically been given carte blanche to conduct a benign military occupation of Europe for its own interests. There's nothing wrong with that, but it makes no sense to pretend NATO is something other than what it really is.
You must have slept through that whole “Cold War” thing.