It's not a sin eater, it's an Amdapori golem that somehow happens to be identical to one. (Note that it remains as a stone statue after the battle is over.)
Also worth noting in the hard dungeon: in the area leading down to what was originally Diabolos' arena (named "the Arrested Darkness") we encounter several "Rift Dragons".
From what they said in the recent live letter about having settled on their plans for Shadowbringers by around 3.4, the idea would have been coming together when the Lost City hard dungeon came out in 3.2 - though I'm not sure how far in advance they'd be working on a patch dungeon.