Casso didn't lie about meeting Gravano. The two did meet, there's surveillance photo of Casso & Amuso with Gotti & Gravano. What Casso said in reference to Gravano that killed his deal was that Gravano sold him drugs. Gravano told his handlers he never dealt in drugs, Casso saying he did would have made Gravano appear as unreliable and open the door for a chance at an appeal from Gotti. The feds didn't want their star witness to be undermined, so they called Casso a liar, and that among his continued criminal acts even while in prison, is why his deal was thrown out. Gravano only proved Casso right, years later.