Micro-triangles are a special case and Nanite still uses the hardware for more reasonably sized triangles.

"Turns out we can beat the hardware with triangles much bigger than expected, far past micropoly. We software rasterize any clusters whos triangles are less than 32 pixels long." - https://advances.realtimerendering.com/s2021/Karis_Nanite_SI...

It's not just micro-triangles, it works for triangles that span multiple pixels. And that's not a special case, that's the standard nowadays, except for games targeting very low-end devices.

The dedicated hardware is nice for general-purpose support for arbitrary triangle-soups. But if you structure triangles a certain way and have a certain amount of density (which you want for modern games), you can specifically optimize for that and beat the general-purpose hardware rasterizer.