I just put the movie in and watched this scene. I will concede to you that it may not be said in a sarcastic tone especially when you take in to account the "smart move" statement." However I stand by the fact that Michael expected it to be Tessio that betrayed him all along. I think FFC put those scenes of Michael and Clemenza in to show the viewer there was some sort of bond between the two. We see none of that between Michael and Tessio. Was it the smart move? Vito told Michael exactly how everything was going to go down. Did Tessio not learn anything from Vito. If Vito was able to predict what was going to happen, wouldn't smart Tessio be able to come up with a plan that wasn't so obvious. And then to top it off you go to Michael with your plan in the middle of his fathers burial. Then you go over to the enemy and shake his hand right in front of Michael. It may have been the smart move but Tessio sure set it up like a complete idiot.