I image that is information that will be relayed in the final part of the Eden Raid during 5.4 which will probably provide some answers but I'd guess a Flood of Light isn't necessarily required for Sin Eaters to come into existence. Sin Eaters appear to be caused by aether corruption when it is pushed too heavily in the direction of Umbral just as Voidsents are corruption in the Astral direction. The Flood of Light and the Void simply makes the creation of such entities easier, but are not necessarily needed.
