You are correct that it is the Death Warden, but it's not because they fixed the bug. The Death Warden you fight in the chest still will not appear on the Veldt. There are actually two different, but identical Death Warden formations(both just with a single Death Warden), however. One appears on the Veldt(and was unused in the SNES version), the other doesn't(the monster-in-a-box). The formation that does appear on the Veldt was the formation used for the Death Warden that you fight in the Soul Shrine in the Advance version of the game, so when you encounter him there, you will finally be able to obtain his Rage on the Veldt.

You didn't have the reasoning right, but you're close enough, so go ahead!