split stitch is typically used as an outline stitch, but it can also be used as a fill stitch if you make parallel rows close together.
when filling a shape, start by stitching the outline. then fill in the area, row by row. for a densely filled area, keep your rows really tightly packed together. if you want a looser, more sketchy filled area, you can stitch the rows further apart.
here's an illustrated visual of the same shape:
hope this helps. happy stitching! 💙