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.