After the pre-patch update 5 man dungeons from TBC or WotLK show the exact same loot in both difficulties(adcenture journal issue I guess), yet the actual loot is still different.

This is correct. In 7.3.5 when they did the scaling changes they merged Normal and Heroic loot together. It's the exact same itemID + appearance, the only difference is quality color and ilvl.

This means if you loot something in normal you get credit for heroic too, regardless if you are on Unique or Completionist.