Hmm, I can't remember the wording "Sephiroth" uses prior to the Temple of the Ancients, but if he definitely claims to be a Cetra then it could be purely part of the act. They did need Cloud and Co. to follow them after all. I seem to remember a lot of the drive to go to the Temple coming from Aeris wanting to know about her past, not the black materia, though I do need to play it through again to be sure.