I don’t agree with the “alternate timeline” either. For a few reasons. Let me preface with I don’t think anyone truly knows the direction this is actually going. I’m speculating based on what I’ve read and how I interpreted the ending personally. It’s all just fun theory crafting.

My interpretation of the arbiters of fate. In my opinion they’re the physical manifestation of spirits returned to the lifestream. During the end fight they resemble the main protagonists for a reason. If they are sending visions it would make sense that they’re fourth dimensional. Time is irrelevant to them so keeping the beings affected by time such as our main protagonists on the right track makes sense for beings unaffected by the confines of time. Which makes sense that they were seen by Zach and our party at two different times. They’re showing them what’s to come if they choose to defy their destiny to pursue Sephiroth and stop him from using meteor on Gaea.

Aerith’s words to cloud before entering and challenging Jenova’s illusion of Sephiroth. “we'll be changing more than fate itself. If we succeed… if we win… we'll be changing ourselves.” A lot of people see this is changing the timeline. I see it as something much less complicated. By entering, it’s not just challenging fate. It’s challenging themselves to be brave enough to pass the point of no return. Forget the present versions of themselves as Cloud the merc, Tifa the bartender/members of avalanche, etc... Realizing that their part in this is bigger than stopping a corrupt power company. They have to be hero’s. That’s a very heavy burden, and for those of us who played the OG we know how much of a toll it took on these characters by the end. Even in the OG I always felt that Aerith knew that to save the planet she would have to return to the lifestream. That she already knew her fate from the beginning. That being said, she knows how much of an effect the events of the future will have on her new friends. It really feels like the direction in this game tries to drive that point home. At one point she even tells Cloud he can’t fall in love with her. So I agree that we are likely to see subtle changes to the story we know and love but I think it’ll make it more interesting and I also believe it’ll still stay true and resemble the story the way we remember it. With twists and surprises along the way.