AP rarely literally means AP, but is a shorthand for various stages of gear you'd have if you upgraded everything evenly. I don't care about your 1.5k if you come with Oath necklace and Destiny ring, that's still story gear. There are certain AP ranges that certain gear level can get you without maxing your gems, and it's faster to type "1.6k" than "upgraded BT+VT accessories, low-to-mid-aransu tier weapon, at least max tiger bracelet, but Hong bracelet preferable and legendary soul"