ACK is a retroclone in the sense that Lamentations of the Flame Princess and the upcoming Dungeon Crawl Classics RPG are retroclones. That is to say that they take the mechanics of classic D&D as their starting point, and then tweak them to suit the style of play the game is intended to encourage. In this case, the focus is on "providing integrated support for play across all levels of a campaign". To put it another way, it's built from the ground up to support things like domain building and that old-school D&D "endgame" people love to talk about.
Autarch, the publishing partnership working on Adventurer Conqueror King, includes luminaries like Alexander Macris (of The Escapist fame) and Tavis Allison (of The Mule Abides and numerous D&D products). The game is currently in its Kickstarter phase but it's evidently well fleshed out already, rules-wise, and also looks like it's going to be top-notch in the production value department.
I recommend checking out the design blog for a better idea of what the ACKS (Adventurer Conqueror King System) is all about.