@Nyaaa
No. You didn't read the explanation. When one dude goes to the other timeline, he merges with himself from that timeline. If the demon king from the previous timeline was Kail, Kail would not be fighting him, because he'd had merged with Kail from the very start. We'd likely have Seran telling the story instead, because Kail ceased to exist.
The others suggested a different theory, in which Kail will become the future demon lord. This is possible, because the future hasn't happened yet.
If this black winged dude is Kail himself, either one of them would not exist, as Kail merged with himself. There cannot be two or three Kails in the same timeline. If this black winged dude is Kail, and the magic does not fuse him with himself, there would be 3 Kails in this timeline, which is the 'Kail from the previous timeline', the 'Kail from this timeline' and the 'Kail who's the black winged dude'. Not possible.