Preparing for the battle beforehand also could greatly weaken him, as there was also the 'friendly monsters' quest by which completing it would deactivate Ozma's Shadow absorption and thus Doomsday would no longer heal him but damage him (along with other Shadow elemental attacks), and that it also allowed the player to attack him with regular physical attacks (which he was otherwise incapable of being hit by).

Either way, this is clearly not meant to be the exact same battle, but much like the Gilgamesh battles, have similar ideas and the basic 'feel' of the fight, given FFXI and XIV are two totally different games with different battle systems and mechanics, it's not surprising. I'm just excited an FFIX boss is appearing, given FFIX is one of my favourite FF games.