Cloud is a world traveler, being in Olympus & all that. He could just have been visiting like Merlin & Scrooge. I always headcanoned that...
A. A Midgar/Gaia world does exist in the KH universe
B. The KH version of the world plot to Midgar/Gaia has already happened off-screen
C. Tifa died instead of Aerith in this universe version of FF7's plot (perhaps Tifa had pushed Aerith out of the way & took the hot herself)
D. Cloud is literally haunted by the memories of Sephiorth & Tifa due to his failure to save her
I had this theory since KH2 & now that FF7 is full-steam on alt histories it might actually end up canon.