What's the point with that quite lackluster story in Endwalker?

[Lauront]

Indeed, albeit in an area that has some manner of aetheric "suppression field". With that said, lorewise LBs were given aetheric descriptions beforehand, and that is for the player jobs. However, given that Elidibus is drawing so readily on prayer in that fight, and they hint at an association between it and dynamis and then with Meteion recognising them as dynamis when you use one, it gets murky. Could be either/or (including for the player characters outside of that fight) but as I say, Elidibus's case is not clear-cut.

Shouldn't she have surviving soul shards then? Her being sundered is just messy. Like, why didn't she suffer any of the same consequences of it that everyone else did? Zodiark becoming increasingly more powerful with rejoinings while she's at a fixed (whole) power level would also make more sense.

Hmmmm it's very messy and they don't explain it in much detail:

Q: Were Zodiark and Hydaelyn completely destroyed? Or there are still Sundered parts of them on other Reflections?
A: To be really frank on this one and to give you a very straight answer here, Hydaelyn was completely destroyed. Similarly, Fandaniel crushed Zodiark’s heart which is his core and that completely destroys Zodiark. There were of course parts of Zodiark in the other Reflections when he was sealed and split across all of them but at the same time of his main body was destroyed, they all would have also faded into oblivion.

So the same answer could be taken for her - my assumption being that the core for some reason was stuck on the Source, and with her being the heart, she was exempted from that treatment. You could argue that not being bound and "heartless", she was able to reform her primal body. I don't think they explained this stuff using the best explanations, but it's what we got. My guess is they didn't want to have to do any further writing regarding their presence on the Reflections.