FAB 3000 is going to export a STEP that looks exactly like the layers displayed (WYSIWYG). Essentially if you create any filled objects in FAB 3000 (like Polygons and Composites) they will export as filled in the STEP. In your sample workspace file, the borders are not filled for your selected layers which is why they're not filled in the exported STEP file.
Note: The Layer that does not need to be filled is the "Border". During STEP Export, FAB 3000 will automatically use the border layer and fill it for all exported dielectric layers.
Note: An easy way to visualize how FAB 3000 will export the border layer is to use the "Real PCB" editor mode.