The WoL is a Sundered Ancient. There probably are more of them in the reincarnation cycle. I also can't imagine that Hydaelyn being able to Sunder beings just came out of nowhere. It might even have been necessary to divide Zoidark into something Hydaelyn could actually seal away.

By the time Hydaelyn was summoned, Zodiark had the aether of 75% of the surviving Ancient population. Hydaelyn's starting aether was much, much less then that. At a bare minimum, she could have started with 25% of the Ancient's aether and that's if all the Ancients sacrificed themselves to summon her. Which we know they did not. So Hydaelyn would have started out with much less aether then Zodiark. She would have had to make up that difference somehow. Splitting Zoidark (and the world) into fourteen pecies that she could seal away isn't the worst way to handle the problem. At least it doesn't seem like when Hydaelyn Sunders beings that she kills them.

And we do know the Ancients who summoned Hydaelyn really didn't want any more death to have to happen. At least, deaths that couldn't result in reincarnated souls. That's why they summoned Hydaelyn in the first place. So I can see them summoning Hydaelyn already knowing they were going to be Sundered when she dealt with Zodiark. Even the Ancients who didn't help them summon Hydaelyn and ended up being Sundered would at least not get stuck in Zoidark but end up in the reincarnation cycle. Which would probably be worth it to some people over having to watch even more people have to sacrifice themselves to Zoidark.

The other thing is... why does people knowing about Hydaelyn's role in all this matter? The only reason people end up finding out about her is when the Ascians stat messing with the Shards and the Source and Hydaleyn tries to make sure a Rejoining doesn't happen. Outside of that, she never seems to interact with the people who are alive and doesn't demand they worship her... or demand their aether... or keep them from being reborn...

If there's one thing Hydaelyn could have been said to fail to do, it was that she failed to Sunder Emet-Selch, Lahabrea and Elidibus. I really want to know how those guys managed to not get sundered. 'Cause if they had been Sundered... it's very likely none of this would have happened at all. We would have the Thirteen Shards and the Source just existing without any outside interference. Instead we have three tempered Ancients who are trying to kill everyone to bring about a plan I don't think they even know will work.