The key difference is that if you go to Luculla too early, you could go back to Cyseal and scoop up the XP you missed. You can't go back to the Island of Appropriately-Levelled-Enemies in this game.
Do we know this is definitely the case? It seems that the Magisters' handy death camp has been mortally wounded and will likely be exposed for what it is once our party escapes; but even without a logical explanation, it's possible we can still travel back and forth.
Unless I've missed the point and it'll be like slaying the seemingly endless goblin imp things in Orobas for 1XP each...
