So, first, since this is not a Paizo product, you're probably better off asking the forum on the site the archetype is found on.
Now, having read the archetype, I have to say I would never recommend playing this. You get some extra AC, paltry DR, bleed resistance, and pointless melee fighting feats (IUS and Vital Strike? Really?) but lose 4 hexes?! 4?! No, that is in no way worth it. I would trade away 9 AC for 4 hexes any day.
Anyway, how it works is by breaking the rules of the game and inventing a new type of AC bonus: the enhancement bonus to AC. See, normal enhancement bonuses are actually piggybacked on other AC bonuses. For example, Magical Armor provides an enhancement bonus to your Armor Bonus to AC, while an Amulet of Natural Armor provides an enhancement bonus to your Natural Armor bonus to AC. They don't have to worry about stacking, because they enhance separate bonuses, they don't apply straight to AC.
This one is probably just sloppily written and intended to provide an enhancement bonus to your Armor bonus to AC, rather than applying an Enhancement bonus directly to your AC as it does now (the way it is written now, you could wear enchanted Silken Ceremonial Armor or Bracers of Armor on top of this tattoo and it would stack, and I am quite positive that is not the way it is intended to work). The enhancement bonus provided is simply equal to the armor bonus provided by the referenced armors (+3 for studded leather, +5 for scale, +7 for banded, and +9 for full plate). It does not carry any other baggage that armor brings--no ACP, no Max Dex, nothing--because it does not provide actual Armor (nor an armor bonus), it applies an enhancement bonus directly to AC.
Weirdly, it says it can be enchanted, which means you should be able to apply an enhancement bonus to it, and that would, depending on your reading, either enhance your Armor bonus (which is 0, like most races with an amulet of natural armor) or to the Enhancement bonus it provides to your AC.
Oh, as a further wrinkle of this poorly written invented bonus: it would apply to both Touch AC (because "your touch AC doesn't include any armor bonus, shield bonus, or natural armor bonus," enhancement bonuses aren't mentioned because they don't exist) and Flat-Footed AC (because "you can't use your Dexterity bonus to AC (if any) while flat-footed," and once again, the nonexistent enhancement bonus isn't mentioned).
So, yes, that's how it works, and even though it's sloppy and the AC bonus it provides is made up and applies too broadly and stacks with things it shouldn't, it's still not a worthwhile archetype because hexes are some of the most powerful class features in the game and giving up 4 of them for this is never going to be worth it.