A very powerful and capable sorcerer so talented that he’s elected to the Convocation of the Fourteen, able enough to avoid being sundered himself (which is another inconsistency that the OP mentioned), but suddenly can’t identify what’s happening or what’s going on? If that’s the logic that some people want to go with then I guess it is what it is.
Well, this is certainly an interesting interpretation of that quote, but to be honest I just don’t see this as Hydaelyn talking about a direct battle with Zodiark. For all we know she could be speaking about how she sundered everyone before Zodiark was summoned because if Zodiark were to be summoned then it would already be too late.
The change was controversial, which is why we see plenty of posts on the forums of people expressing their discontent with the events surrounding the sundering. Labelling something as a retcon can also be used to call out a writer for changing the story around in a way that's more convenient for the writer, which is exactly what I think happened with EW.
Retcons can be used by writers to handwave away details that are a nuisance to work with, and a somewhat knowledgeable writer can disguise a retcon through "newly received information" or some other "unexpected occurrence" that changes details of the story. Yoshi P already admitted to doing this for parts of the story during the Endwalker Q and A session: "when you're making this world you'll just sort of sometimes ignore things that are inconvenient" (4:25:35). https://www.youtube.com/watch?v=WRpdIL7_NII
I believe the OP already mentioned this, but it seems that the retconning of ShB's story was a thinly veiled attempt to make things more convenient for the development team. Maybe the painstaking process of creating a detailed metaphorical or literal cutscene that showed a battle between Zodiark and Hydaelyn would have taken too much out of the allotted time for developing the game, so they just changed the story around a bit to show that the Sundering was something she did by herself.
Remember that the release of EW was already delayed for two weeks because Yoshi P wanted to edit the cutscenes on Ultima Thule, and they were already behind schedule and on crunch time. Could you imagine how much more of a delay it would’ve been if they had to create a cutscene (metaphorical or not) where Zodiark battles Hydaelyn?
What’s more is that it almost seems like SE tried to gloss it over by writing the Tales of the Dawn entry that reconciles both ShB’s and EW’s version of the Sundering. It’s almost as if the writers thought: “Hopefully people will forgive and forget that we didn’t include the bit about the battle between Hydaelyn and Zodiark in the EW MSQ if we write it here.”