Yeah just because Itachi says/thinks Madara is invincible, that doesn't make it so. Maybe in Itachi's eyes he is.

My Guess is that Madara messed pretty bad with Itachi's head when he was seven (or so), and Itachi has a Madara God-father comlex. No doubt Madara is unique and really powerful, but in the end the Konoha gang will defeat him (so much for invincibility).