Dark and Ghost interchangeable for attacking?

You're reading too much into it. They stated the simple fact that Dark and Ghost have the same attacking type effectiveness, that's all.

Also all 3 tier 3 bosses that you mention have a charge move that specifically punishes Tyranitar. These things are more complicated than just stating what dark is resistant against.

TDO is not as important for gym offence. You need to take gyms down before the defender notices and feeds, so speed is paramount. This also ties into healing item efficiency, taking the gym fast is many times more efficient than fighting an extended battle against berries or giving up after a few fights.

When you factor in enemy moves there are circumstances when Tyranitar is a better option than ghost-types and vice versa. Dark Tyranitar will continue to be vital for the raids it's best suited against and will continue to be the king dark-type attacker until Yveltal [Darkrai too but there's potential we only receive one], it just faces stiff competition with another type.