Yeah alpha masking is inefficient under Nanite, but as they explain further down its handling of dense geometry is good enough that they were able to get away with not using masked materials for the foliage in Fortnite. The individual leaves are modelled as actual geometry and rendered with an opaque, non-masked material.

https://cdn2.unrealengine.com/nanite-in-fortnite-chapter-4-t...

https://cdn2.unrealengine.com/nanite-in-fortnite-chapter-4-t...