The language "more stable" as applied to "more straight" goes back to Fred Morrison and Steady Ed Headrick (that was the idea of the ridges on the Wham-O disc). It made sense then, and has been used ever since. I'm not sure when the misuse of the term began, although I wouldn't consider DGCR to be an authority on anything. Overstable means it hyzers left, understable means it turns over right. Of course these depend on the speed, the amount of wobble (OAT), the angle of attack, headwind vs. tailwind, etc., which is why we speak of high speed vs. low speed behaviors.
Some discs are very stable both at very high speeds and very low speeds, I've seen some pros throw a Buzzz ridiculously far (450'+) without flipping one iota, and then finishing perfectly straight...that's as stable as stable gets!
Distance Drivers: Barsby Boatman, Glow Champion Katana
Fairway Drivers: Pinnacle Rival, Icon Patriot, Protege Rival, Elite-X Cyclone
Mid-Range: Ghost/Wasp, Z Buzzz/DGA Squall, Z Glide
Putt/Approach: Star Classic Roc, Icon Clutch, Protege Clutch