No, not really. Understand what "canon" means before you go throwing it around
"Canon" isn't really an applicable term to subjective arguments (and Itachi didnt really say Madara was the greatest, assuming you're referring to where he calls him an "invincible immortal". At best, you can justify that statement means he was calling him the strongest, power-wise. It has 0 application to his morals, etc).
For example, if I said that the Raikage can't evade Amaterasu or Minato wasn't Naruto's father, I'd be wrong, because those are two things that are canonical and cannot be debated.
However, things that aren't 100% objective are debatable, such as the " greatest" Uchiha. Itachi could have wanked Madara on every page in the manga, and I can still say that he wasn't the greatest and be right. Because to me, being the greatest might not mean being the strongest, but about what actions you did, etc.