Android-x86 does have the open-source nouveau driver for NVIDIA GPUs
(community developed based on reverse engineering), however with newer GPUs
(from GTX 9xx desktop and GTX 965M+ mobile) the GPU will be stuck at its
lowest power state and clockspeeds, so it will likely perform at about the
level of integrated graphics, if not worse. Also given the lack of support
from NVIDIA officially there may be a chance of it not working right even if the
hardware should be supported in theory.
For Android-x86 specifically that only applies if the display is directly
connected to the NVIDIA GPU, which is often not the case in laptops, where the
display is actually connected to the integrated Intel/AMD integrated graphics,
and the NVIDIA GPU needs to go through the integrated graphics to display
anything. In those cases Android-x86 will only use the integrated graphics,
and for Intel ones at least that should work fine. Or, if said laptop happens
to have a switch in the firmware to disable the integrated graphics completely
then Android-x86 will attempt to use the NVIDIA GPU, but again caveats apply
as on desktops.
In short, it's just a matter of luck really, you can thank NVIDIA for that.