There is 1 big different between sin eaters and voidsent, voidsent are not just instinctual creatures unlike sin eaters which have not shown any semblance of thought
Vauthry/Innoccence is a weird exception as is implied it is half human
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Sin eaters have lost anything that would be considered a person when they have been turned for awhile
Tesleen being the only sin eater to speak and Lyna specifically mentions it is due to being newly turned
yet Voidsent very much can plan, manipulate and are not just instinctual beings some are but the higher ones are not, so it might be possible that reverting the flood of darkness might force the higher beings to change back into spoken races (which would be another nightmare scene of lovely pain).