I'm honestly 50/50 on this leak, it's plausible and the reveals are actually good. It's also plausible this is someone making very grounded, educated guesses and assumptions utilizing data already known.
But keep in mind, FFXV's plot, gameplay and development status got leaked relatively early and no one thought it was real given what it included. I wouldn't be surprised if this was true, and I wouldn't be surprised if developers felt inclined to use the FFVII Remake to include discarded concepts they didn't have time to incorporate in the OG.
It's something worth paying attention to. When E3 roles around, if demos and the trailer line up with what this dude posted well.. That'll be your answer on if this person's legit or not.
I also hope that the game is split into 2 parts because that would honestly make the most sense and give the best sized content for each part.