......( have not open your file )
DXF is usually a 2D (or a 3D ) format of lines & arcs, & may not hold solid shape data
( ie the lines may form a cubic shape.....but the cube solid is actually missing )
polygons is probably a collective term for any solid shape
If your AutoCAD drawing contains solids.... try exporting as a STP (step) file... or another format that supports 3D shapes