They do. A workaround is to export a 2D image from SketchUp in ortho view. Open a fresh Sketchup file and import that image into Sketchup as an image, position the camera so it is looking straight at the image (use a standard view) then apply the Diffusion generate. Diffusion will still recognize the distinct elements and apply styles to the image.
Example file:
Untitled.skp (3.5 MB)