Is there any way to export a floor plan (being a section) to a deg, where the walls in section show as black fills in autocad?
I think you mean to dwg. The section cut isn’t recognized by the dwg translation.
Anyway the free web version doesn’t have dwg export
One thing that could help with dwg is to have the section cut export on it’s own layer (SectionCutEdges). In AutoCAD or whatever, you can isolate that layer and fill all the enclosed shapes. I don’t know ACAD to know if there’s a quick or automated way to do that.
If you make a group from slice in the contextual menu you can make a 2d plan from the section in SketchUp, put a face on it, painted black, and transfer the black fill with the edges exporting 3d dwg plan view. Doesn’t take much. Only OP is using Free and there is no dwg.