If Psychic was super effective against Dark, or even neutral against it, the triangle wouldn't be complete, right? But since Dark resists Psychic, it is. Instead of it being "not very effective" though, it's completely ineffective. But does that really matter? Dark still resists Psychic, so it's still a triangle. Again, such a minor difference is not something to get all butthurt over.
My point is that resistances matter just as much as anything else. It's not a triangle.