Section slice only exports the cut geometry, only what the section plane touches. If you want the whole scene you need to export to 2D scene and select .dxf as the format.
Slicing a Model to Peer Inside | SketchUp Help
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It worked! I should have exported it as .dxf instead of .dwg, grad to know the difference too. Thanks
No difference there. The contents of a DWG and DXF file are identical. The difference lies between exporting the section slice (lines formed at the section cut plane) and the section view.