If Kreia trained the Assassins in the ways of Nihilus, why does she explicitely state in the game that technique must die, together with the assassins? Doesn't make a whole lot of sense.
And yes, Sion and Nihilus do use tact, not brute force. It's not only because of Kreia. Everybody states that ingame.
It's a very nice theory, but unless I see some ingame proof, I am not going to believe that's how it all went down...