I believe it does open the old instance, though. To be able to craft the gear you have to enter the instance and do more secrety stuff. And it apparently requires a lot of RNG luck and tons of gold to make the gear
Naw, for Naxx it’s still the Wrath instance. The extra portal you unlock in Eastern Plaguelands still sends you to the Wrath instance, and all the trash and bosses still use the Wrath loot tables. All the stuff related to the ‘secret’ crafting was just added to it.