Nocturne's W doesn't block Poppy's Passive ability?

How come Nocturne's W [Shroud of Darkness] doesn't block Poppy's Passive [Iron Ambassador]?

His W as described - when activated it blocks a single enemy ability and increases his bonus attack speed from his W.

Poppy's passive - When available, throws her buckler to do damage. This is considered an ability, no?

Why is Poppy's passive spell excluded from being stopped by his shield?