Why is dragon-steel-fairy treated the same as fire-water-grass in discussion?

Fire-water-grass cores are good because they cover each other perfectly; fire beats grass, grass beats water, and water beats fire. It's a cycle. A core.

However, I've noticed that in the same breath, people will sometimes talk about "dragon-steel-fairy cores". See the "classic cores" section of https://www.vgcguide.com/cores-and-modes as an example.

I understand that these are all excellent types, but their dynamic is nothing like fire-water-grass.

Steel beats fairy. Fairy beats dragon. Makes sense. But dragon doesn't beat steel; in fact, steel beats fairy and also resist dragon. This means that in this "core", steel becomes the superior type, whereas in fire-water-grass, the three types have equal weight. Dragon-steel-fairy is more of a triangle, with dragon and fairy at the bottom and steel at the top.

Isn't steel-dragon enough of a core by itself anyway? Dragon loses to dragon, fairy, and ice, and steel resists all of those. Fairy doesn't need to factor into the discussion at all besides producing a dragon immunity and a steel weakness. Surely a core of dragon-steel makes more sense than dragon-steel-fairy.

I just don't see how dragon-steel-fairy is a core in the same way that water-fire-grass is. When did this proclamation start? Why is it repeated to newer players so often?