The moment you start doing 3D rendering, the GPU goes into high power mode, so even light GPU work will see the GPU drawing more power. It is entirely possible that Apple has more advanced GPU power management (they do need to use advanced visual effects while delivering good battery life on the phones), but I am not aware of any details. Usually, as soon as you start using the 3D API explicitly, your power consumption jumps up. Would be interesting to do some more in-depth tests though...
Anyway, my point is that the M1 GPU draws 10W at it's maximum. So you can be pushing it pretty hard and still get a decent lifetime on a 50Wh battery, especially if you have some power optimizations. A 30-40W GPU on a 100Wh battery though? Not so much.
ZBrush doesn't use the GPU at all, even viewport rendering is done using the CPU (and a lot of cheating, approximations, and only updating the parts you're actually sculpting) and it's how ZBrush can work with poly-counts in the tens / hundreds of millions, as opposed to GPU based 3d tools like Blender.