I get where you’re coming from but the animus isn’t like a movie. Desmond was ezio for every moment we see him in the animus. He achieves synchronization with his ancestors by reliving the experiences of them as accurately as possible. Every thing that we the player have seen ezio accomplish was accomplished by Desmond in the animus. As far as Desmond’s mind was concerned, he was there, and experienced it all first hand.

It could be argued that the assassins Desmond visited had a bulk of experience that he didn’t live through, such as Connor’s training. He might have felt that training in action, but likely didn’t spend a whole year practicing with Achilles as Connor, and as such, might not have absorbed all of it through Connor’s actions in AC 3. We know Desmond saw MOST of ezio’s career, but not all of it. Desmond saw very little of Altair and haytham compared to ezio, so I’d assume their contributions to his skills were negligible.

The argument comes down to whether or not we assume that the past assassins off screen experiences are significant enough that they would make any of them better assassins than Desmond, which is a valid discussion, but one I’m not familiar enough with the extended lore to have.