It seems that I misread the settings, they are Named Ambient and not Ambient Occlusion.
After reading on https://developer.nvidia/vxao-voxel-ambient-occlusion I did a quick test. And yes, if you enable r.VXGI.AmbientOcclusionMode the diffuse lighting part
of VXGI disappears. However I was still able to get an AO effect, but I am not sure where it comes from.
Edit: In both images, r.AmbientOcclusionMode is set to 0
In image you can see the pitch black areas. I removed the skylight to make it visible. VXGI Ambient Setting is deactivated.
In the next image the Ambient Setting is activated with color set to white. You can see that in the corner it shows the typical occlusion effect of AO.
NOTE that both HBAO+ as well as UE4’s SSAO are turned off/set to 0.
Where does come from?