smoothing groups are not used to recreate any vertex normals.
they are only used to define the bounds of interpolating the provided vertex normals during rendering.

without smoothing groups, models with large angles between polygons get abnormal shading, even when you use the provided high-quality vertex normals.
you need smooth groups in the OBJ file to render them correctly.

smoothing groups have nothing to do with recreating vertex normals in any way.
i think you're mixing up the concept of using smoothing groups in a modeling application,
and the transport of data in OBJ.

Radiance