Yeah, I definitely always got the impression that it was intended as a fake ending. The buildup to the Floating Continent makes it seem very much like it's the end of the game if you don't know anything about the story. It's even listed as one of the examples for Disc One Final Dungeon on TV Tropes. Then again, the TV Tropes entry implies that there was a map for the World of Ruin somewhere in the packaging which would have been a giveaway.

I believe on my first playthrough I knew ahead of time that something bad was going to happen to the world before the end of the game because I was playing on my friend's cartridge the first time I played the game and he had a saved game with a completely different world map which I accidentally opened before I had gotten that far. So when I got to the Floating Continent I knew it couldn't be the end, because nothing bad had happened to the world yet. However, I still definitely got the impression that it was intended to be a decoy climax.