Just as added information, for future folks who find this topic. We can actually export individual SurfaceBody objects (a SurfaceBody object can be either just a surface or a solid). We are just very limited on the file formats we can export them as...directly. We can use the SurfaceBody.DataIO property to access a DataIO object, that we can then use further towards the export process. Then we can use the DataIO.WriteDataToFile method to export the body out as various variations of "ACIS SAT ..." or "ACIS SAB ..." file types. I am not familiar with those file types, so I never used this process myself. There seems to be about 10 variations of that 'format' that can be specified when exporting like this. We can get this information ourselves by using the DataIO.GetOutputFormats method (which is how I originally got it). Then within its 'storage types', all variations could be stored to file, but one variation could be stored to a file or 'Stream'.
Unfortunately, there was no option for exporting directly to STEP or STL file type.
Edit: I just did a little searching, and found the following:
I believe the actual file extensions would be ".sat" & .sab".
Links:
https://www.spatial.com/products/3d-acis-modeling
https://en.wikipedia.org/wiki/ACIS
Wesley Crihfield

(Not an Autodesk Employee)