Yes, it does add AC, but...
Not to anyone wearing heavy armor already.
The Armored Kilt in the Adventurer's Armory only says this:
Armored Kilt: When you add an armored kilt to a suit of light armor, the set counts as medium armor. Likewise, a kilt and medium armor counts as heavy armor. Adding an
armored kilt to heavy armor has no effect.
This suggests that the Armored Kilt is nearly useless, but this description is only to "update" the Armored Kilt from Pathfinder Chronicles Campaign Setting, suggesting that anything not mentioned is not changed.
The PCCS has a very different description:
Armored Kilt: [fluff] An armored kilt can be worn separately as light armor, or it can be added to other suits of light or medium armor. Adding an armored kilt increases a suit of armor’s armor bonus by +1, but it adds
15 pounds to the armor, lowers the maximum Dex bonus by 1, and increases the armor’s weight category (from light to medium and from medium to heavy). Adding an armored kilt to heavy armor does not provide an armor bonus increase.
The classes that would find the biggest advantage in this are full Arcane Spellcasters. The Armored Kilt is the only piece of armor in the game that does not come with an arcane spell failure chance. This means that wearing it does not have a chance of causing any spell with Somatic components to fail.
Since having no armor does not come with any DEX penalty, wearing no armor and an Armored Kilt actually allows you to use your full DEX bonus, regardless of what your DEX bonus is.
Do note that it cannot be enchanted or masterwork-ed separately from your regular armor.