From the reasoning they gave (at least from what ive seen), they effectively cherrypicked shit that affected some of his weaker avatars and applied that to all of his avatars across the board as well as his true form, when his stronger avatars have numerous feats of being outright unaffected by narrative/history hax. Its very clearly established that the shit they claim would kill the scarlet king just flat out does not work on him in-canon.
To put it into perspective, in order to retcon the Scarlet King from existence, you would have to not only retcon the concept of antagonistic forces from all works of irl fiction that exist throughout all of irl humanity, but retcon irl humanity's capacity to conceptualize evil, and even then thats not likely to work as the Scarlet King is formed not just from humanity's concepts, but beings that exist even beyond human conceptualization, as well as concepts that cannot and will not ever exist. And even then thats not likely to work as the Scarlet King is not just humanity's concepts, but all beings that exist even beyond human conceptualization, as well as concepts that cannot and will not ever exist. And then on top of that you have shit like the scarlet king existing in a state of meta-nonduality, where he both does and does not exist everywhere at every point in time at once, so there wouldnt even be a point in time to retcon him to/from, because he has always existed and also has never existed to begin with.
They went with quite literally the most conceptually impossible reasoning imaginable for a way to defeat the scarlet king. The avatars? Sure, you can argue Clark can beat most of those. But he is not beating the True Scarlet King.