Maybe you should play the game. I understand that the games they come from are VII and VIII. I'm not an idiot. However, in the universe of Kingdom Hearts they are from the world of Hollow Bastion. This is fact.

He didnt want to start over. If he wanted to start over, he would have left everything and forgotten about the Heartless. He didnt want to be called Squall because he was ashamed of his failure. You can argue with me all you want, but it's in the game.

Nomura, the creator himself, has stated that it's a different universe. At the end of the game, Squall, Yuffie, Cid, Cloud, and Aerith are all reunited on their own world... Hollow Bastion.