It may have been taken out to conserve RAM...but it doesn't seem like that would be a huge saving
s.
Because the boss fights load, I don't think that space was even close to being an issue and NEC Ave's ports of this game are high quality for the time.
So the maximal benefit is a reduction in flickering during a moment when 17 sprites (including horizontal bullets) would appear on a scanline...
I guess that's better than nothing, but it's hardly "supporting" the SGX hardware.
Since the flicker was the only imperfection left in the port, it's all that was needed. It's not like the existing dedicated SuperGrafx games ran in 512 x 240 or anything.
It's one thing that Daimakaimura doesn't use the arcade resolution, to keep rom size down, and the color is poor by PC Engine standards... but it actually has flicker/drop out in places that the PC Engine shouldn't.
That's a bad example of making use of the hardware.