Nanite can actually do trees now, and Fortnite is using it in production, with fully modelled leaves rather than cutout textures because that turned out to be more efficient under Nanite. They talk about it here: https://www.unrealengine.com/en-US/tech-blog/bringing-nanite...
That's still ultimately triangle meshes though, not some other weird representation like NERF, or distance fields, or voxels, or any of the other supposed triangle-killers that didn't stick. Triangles are proving very difficult to kill.
My understanding is that while they allow masked textures, the geometry is still fully emitted by Nanite. That means you still need to start with a mesh that has multiple leaves baked into a single polygon plane, as opposed to starting with individual leaf geometry and then that is somehow baked automatically.
This illustration from the page you linked to shows that as well:
The alpha masked holes move around, but the polygons remain static. That means if you draw a tree with this, you still have the full overdraw of the highest-poly mesh.
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.
That's still ultimately triangle meshes though, not some other weird representation like NERF, or distance fields, or voxels, or any of the other supposed triangle-killers that didn't stick. Triangles are proving very difficult to kill.