It does not state that Casso Himself Kill Demeo. It does state that the Gambinos reached out to Casso because he had the Gemeni twins in his back pocket and they obviously could get close enough to kill Demeo.

All of that if factual, Casso did have the Gemeni Twins in his back pocket and the FBI believes that the Gemeni Twins are responsible for Demeos murder.

the only thing no one knows for sure, is if the Gambinos reached out to Gaspipe.

Many think that Nino Gaggi got the order from Big Paul and may have actually shot Roy first and the twins finsihed him off.

2 things are certain though:

1. Casso did not kill Roy himself
2. Kuklinski did not kill Roy as he has claimed.