I will try to double check with Eric Chahi but this is so much more efficient that it has to be the way he drew polygons (instead of drawing into a scrap buffer and blitting four times, he used BPL*MOD to draw and blit only once).
If you do several blits, one per bitplane, you get to use the same mask data, but when using interleaved data the mask needs the same shape data * number of bitplanes involved.
It's still a pretty small price to pay compared to Atari ST/ZX Spectrum preshifted bitmaps.
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.