Now Tessio and Clemenza both knew Carlo was a traitor who set Sonny up
Nobody knew for sure, let alone Tessio and Clemenza. It was only when they knew it for a fact when Carlo admitted it to Michael.
at the end of The Godfather Connie says to Michael "You blamed Carlo for Sonny...
[/i]everyone did[i]" (emphasis added). Tessio and Clemenza had to know that Vito naver gave Carlo anything important to do, and they knew Sonny beat him up for what he did to Connie. To see him being named Michael's right hand man could not have made them feel confident.