I have a feeling that Isli is wrong again and Paula is right all along. After Rin took out Nickel, she started thinking about who Over M might be and considered the possibility that it might be... someone. That might imply that she knew something about the potential identity of the demon lord that the younger Idaten, including Isli, are completely ignorant of. Isli assumed that the elders fucked up and accidentally created the demon lord. However, the elders thought during their time that there would be a time when they would pass and Rin would have to take over. That means they had some expectation of mortality. It's also been established that the elders had time on their hands and that the demons as a whole were never a threat to the Idaten. Plus, if we believe the backstory behind the seal, it doesn't make sense that they would be able to physically corral all the demons into one spot but were not strong enough to wipe them out right there and then. Even corraling cockroaches was tough work for the most powerful young Idaten.
Maybe Paula was right all along and 800 years ago, after who knows how many hundreds of years of being genocided by the Idaten, the demons' wish for salvation from the Idaten manifested in their own Idaten, and that was what the elders had to sacrifice themselves to seal because they simply couldn't beat another immortal Idaten.