So in effect, they were setting at trap for Tessio as well.
Michael stated several times in that meeting that they were to be patient, that there were things he couldn't share right now, that they could form their own Families "..AFTER the move to Nevada...". Michael & Vito knew that someone in that room was not going to be able have faith & stay true to their Don. It was the nature of the business they were in.
All they had to do was wait & see who would turn traitor out of imagined necessity.
Kewl.
Apple