So, from what I'm gathering from FFVII:R: spoilers:end of spoilers
Sephiroth came into the past not only to get a second chance at godhood, but also seems keen on keeping Cloud around as his insurance policy in case he dies again. After all, the creators and various supplemental material have stated that the reason that Sephiroth hasn't lost his ego and dissolved into the Lifestream is due to his extreme hatred of Cloud giving him a focus to maintain his ego. I'm assuming that this future Sephiroth originates from the epilogue of Final Fantasy VII (which takes place 500 years in the future IIRC), long after Dirge of Cerberus took place. So, he's trying to feed Cloud half-truths and propaganda that the Planet is going to end so that he'll be easier to manipulate.