You're assuming Casso is telling the truth.
Gravano said the same but i agree not exactly the most reliable informants (if there is such a thing) .
How would Gravano know about a plot to kill Gotti unless he was in on it?
I'd guess it was what he heard on the street. In all honesty though i think i may be wrong about Gravano, but i'm pretty sure i read it from another source confirming that Marino and Falla were in on it. It may have been Montiglio, again not a very reliable source nowadays,but i definitely seen it somewhere other than from Casso.
