We don't know the specific nature of the temporal fuckery at work here. People are taking speculation as fact when the established facts are quite scarce.
Is it confirmed Sephiroth is from the future? No.
Is it confirmed Aerith is from the future? No.
All we know, is that Sephiroth, and to an extent Aerith, have precognitive knowledge. How they've gained that knowledge is a mystery. There are numerous ways that could happen that does not involve literal time travel taking place after a subsequent story, so making the assumption this somehow is fact and it's all a sequel, is erroneous.
It could be a repeated time loop of FFVII's events, precognitive knowledge bestowed from the Lifestream, astrology, tarot reading or anything in-between. Saying it's a sequel because some people know stuff and newly revealed entities have come from the future of the planet.. Doesn't make it a sequel.
Hell, Nomura could really tighten his trollin' belt and make it to where those future visions outside of the final battle were merely visual cues for the audience that don't even exist in-narrative. Like he did with Birth By Sleep and having Terra supposedly see visions of Riku's future when he did the rite of keyblade succession on him. Everyone thought Terra carried pre-cognition until Nomura said "Jay-Kay!" in the subsequently released Ultimania.
Don't leap to conclusions especially after a purposefully ambiguous conclusion.