Yeah no. It's the same story with some tweaks. Not a sequel.No it is litterally a sequel to Advent Children.
The Sephiroth you see in Part 1 is Sephiroth from Advent Children. The fates at the end are from Advent Children. Sephiroth is always there to delay your actions to change the timeline.
For example in the begining. You encounter Sephiroth the first time you should have met Areith in the streets. He delays it so the Fates attack Arieth so you see her and can meet her. And this goes through the whole game. They also confirmed it that it is a sequel. And the reason why it is calls Remake is because Sephiroth remakes history.
People back then complained that there was no part 1 in the title. But it is not a Remake it is a sequel continuing the Story after Advent Children.
if you want the full dose. Here is some of it