The Sephiroth you fight at the end has his wing feathers which are from AC, knows all about Cloud and even asks him to join him in defying destiny, shows the exact effects of meteor destroying midgar and makes sure Cloud focuses his hatred on him.
There is no possible way this isn't the real Sephiroth who was defeated. He shouldn't already know what Cloud is capable of.
The wing is a product of "becoming a monster". Angeal and Genesis both grow wings after realizing the truth and "ascending." Sephiroth never got the chance to use that form in FF7 since he was immediately killed by Cloud. Plus that world and his body are an illusion.
Sephiroth knows all about Cloud. Cloud was the one that killed him. Of course he knows what Cloud is capable of. He's always taken special interest in fucking with Cloud.
He asks Cloud to lend him his strength as a part of Reunion and also because the new interpretation of his goal is he wants to "save" the world from its fated end by having it all join with him.
There's nothing about that Sephiroth that can't be explained without it being a time travelling Sephiroth that knows the plot.
They did not. It was SO bizarre. The entire Chapter 17 I kept waiting for someone to say, "Hey...we gonna talk about that guy with the big fucking sword a moment ago?"
Sephiroth's existence isn't too alien to the planet when compared to any other Soldier. He has one minor difference between other Soldiers, but in the end, it's Jenova that binds them all together.
By the way, are OG 7 spoilers cool for here or should we mark them?
Sephiroth is different in that had Jenova as part of his DNA since before birth so its more part of his being. Regular Soldiers just get injected as adults, so it's more like a virus than a part of their bodies. Also, after he died, he merged with Jenova's head and essentially become Jenova. He is the antithesis of the Cetra.