The algorithm uses the fill object's bounding box, which are always rectangular regardless of the objects rotation. To answer your question directly, yes, the algorithm would double the covered area:
If you need precise coverage calculations for rectangles (or other polygon) that allows for rotation, you'd need to break the entire area into triangles based on each rectangle's vertices, and test if each triangle overlaps the fill object. It's exponentially more difficult, though not impossible.
If you only need semi-accurate calculations, you could do as others have suggested and create a small sprite, or high-density tilemap, and loop through the entire area checking for overlaps. You'd probably end up balancing accuracy vs. efficiency.