Michael doesn't want to tip anyone off.
Just like poker.
If I KNOW that you're bluffing, I'm not going to let you KNOW that I know. So he thought Tom might be a traitor too? No. Not at all. But if he tells Tom to cancel the meeting with Tessio and Barzini and Tom does so, then both Tessio and Barzini would have been warned beforehand that Michael was onto them. One or both of them might have escaped the trap that Michael was laying out.
"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming
Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.
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