That feature requires getting pyopenvdb installed, which can be a headache, and I never really updated the README with examples, but it does work. There is one example script:

https://github.com/fogleman/sdf/blob/main/examples/mesh.py

You basically just say:

f = Mesh.from_file(path).sdf(voxel_size=0.25, half_width=1)

Then you can operate on `f`.