Yes that sounds right - Zinc uses 3d acceleration properties of your pc. It's basically (I'm half guessing here so apologies if wrong) intercepting 3d instructions in the original game that would have been sent to the graphics processor, converting them to directx/opengl and sending them to the pc gfx card. So it does not have to emulate a complex graphics chip, making the game run faster etc..etc... But its not 'true' emulation and I'm guessing not guarenteed accuracy.
Mame would always emulate the complex graphics chips completely, and then run the original code the on the emulating gfx unit..... so in theory perfect, but much harder.
Is this right?