You'd use a single multi-bitplane interleaved fill-blit if the background surrounding it did not need to be preserved (eg. no mask requirements as per Flow - demos tend to do this for its speed).

I'd imagine Another World creates an off-screen single-bit deep bitmap and uses that as a mask. I sent you an email if you'd like to explore more, however the original programmer probably just remembers what he actually did.

Also note that non-overlapping polygons can be grouped together in both approaches by line-blitting with XOR destination (adjacent polygons share an edge and so the edge is XOR blitted twice); so there are substantial speed-ups to be had.