Knowing the weakeness of a Lunarian will play a huge role for the battle, Sanji and the others will used it to beat the Seraphims.
King took 3adv CoC+adv CoA moves of Zoro, let see how much the Shichibukai Seras can tank without flame off, S Shark already took a DJ with flames off, means they not end as fodders even with flame off.
I think this whole weakeness plot also could reveal how to deal with them if they decide to not go flame off. I mean why they should, staying in flame on and spaming attacks is the best way to not get hurt right? Their must be a other weakeness, outside of the weakeness with flame off. I mean if the seras decide to not switch again and stay in flame on, the strawhats wouldn´t be able to hurt them.
Adv CoC can probably do the job, but it not explain how the entire race got killed in the past, I suggest their is something else, Vegapunk should also know it, surprise that Shaka didn´t mentioned the weakeness early, I mean Zoro know it but Shaka should know the weakeness to right?
If the Lunarian have not any other weakeness,only the one with flames off/on, they are massive op, lunarian dna is build different.