I really think that Tobi was telling the truth and Sasuke's reactions toward it just shows how he have successfully manipulated Sasuke.
If you think back of the conversation, Tobi actually started about how much Itachi was just trying to protect him. And of course, after realizing that Sasuke just killed his bro who after all this time just loved him and protected, GUILT must've clouded his mind so much that he thought that the only way to make up with it is by killing Konoha, who have enjoyed so much peace in exchange of his brother's death.
On another thought though, that guilt could also trigger him to actually follow Itachi's wishes to make things up with him - so, he probably quickly thought of playing along with Tobi for now, and go against him in the end. Just like what he did with Orochimaru.
The Itachi-Sasuke bro relationship flashbacks actually shows how much these Uchiha brothers love each other. These words of Itachi have been clinging on the back of mind in all those times when they both hunt each other:
"We will always be brothers. Even if i would have to be an obstacle for you, I will always be there for you."
Those words will certainly hunt you with guilt if you were Sasuke who just killed your brother with a definitely wrong reason.
