The Searing & Cauldron of Cataclysm

I'd be inclined to say it can't be the Giganticus Lupicus reference, since that's presented as prehistorical information. Counting that as "recorded history" would be equivalent to counting the last sixty five million years as "recorded history" because that's how long ago the dinosaurs died out.

Without that, the oldest record we know of was the arrival of the Forgotten, which means that the gods predate what recorded history we know. Now, it might turn out that Charr recorded history goes back further, but even they reference Melandru's involvement in the creation (terraforming?) of the world as a legend rather than as recorded history, so I'd say there's nothing that conclusively rules out a divine origin for the Cauldron.

Still no comparison shots of every cauldron ingame?

Every cauldron, aside from perhaps size, is essentially the same. I've not noticed any difference in the cauldron found in the Desolation, nor in Sacnoth Valley, which would have been their chance to make some alteration to their appearance if they had done so.