I'm more inclined to believe that Saazbaum didn't believe that the Terrans had any way of defeating a Kataphract without someone helping them. Since Slaine is a Terran, knows some of the martian secrets, and was assigned to the killed moron, he'd be suspect number one when a racist martian noble is on the investigation (as well as another one being the victim).
It's just fortunate for him that not only was Slaine very likely involved in the actual killing, he is also suddenly quite knowledgeable about their whole conspiracy. Saazbaum basically had every suspicion confirmed.
What was bullshit was that he already knew enough of that in order to dissuade the Emperor from looking into what Slaine was actually saying. He shouldn't have been able to give such a convincing argument without learning what Slaine knows before he said it.