Astarion's armour is supposed to be Padded Armour, which it doesn't particularly look like, true -- though gambeson/aketon based civil clothing were very popular in the late medieval/renaissance times -- but it certainly looks more appropriately like armour than either of the pictures dressups above.
So please. Think again.
Excuse me? It's a game. It does not matter whether it is similar in your understanding or not, it is still armor, it has stats. This suggests that armor of this style is possible in the game and it will still be LIGHT ARMOR that suits rogues, bards, mages, or someone similar. The fact that it "doesn't look like that" seems to be your personal problem of perception versus facts, which are already in the game. And I want to tell you that at this stage, Wyll's armor is also very similar, it's just clothes, there are no "protective" elements, and yet statistically it gives protection. Thank God you've never played Korean games, you'd be surprised what kind of clothing can GIVE YOU PROTECTION. Because in many games, the design doesn't affect the armor stats. And design is important to me.
You write that such armor should give AC 0, but in the game you already have such armor that gives AC 11. Fact.