In that context Michael also notes that all his "people" were businessmen, implying that the hit attempt could be "business not personal." Of course Fredo's motivation was definitely personal and not really related to business (other than the pipe dream that he could be a good Don). I don't think Michael did not suspect Fredo, but by implying that the traitor could be taking Michael out for purely "business" reasons, he was hiding from Tom his suspicion of Fredo, and sending Tom a clear message to keep a close eye on Rocco and Neri in his absence.