Street Fighter vs. Soul Calibur

First of all, Gouki did use KKZ to sink the island. Secondly, despite what some sources say, he isn't completely drained of power after using it, he wasn't Shin when he did it, and he just fought Ryu to a stalemate, logic tells you that. Also, SF will win this, mostly due to the weakness of a lot of the SC cast. The reason why the weaker characters like Xiangua, Kilik, and Maxi get victories over the powerhouses like Nightmare is because of plot devices, they are triple teaming him, and two of them have plot device weapons that are effective against NM and Soul Edge.

And again, where is it said that was the technique he used to destroy the island?

A few sources, I would have to find them.

Wiki and a few other sites like that say it. Nothing official as far as I know.

Kongouko-Restuzan did not even exist during alpha for godsakes

^ is also a truth.

There is NO proof whatsoever that he did the move on the island.

The technique did not exist at the time and nothing says it was the move he used.