How come the miniDSP SHD is not in the list of Volumio compatible DACs while it is highly referred to in the equipment user manual ?
Presumably because no one from the Community has reported it as such. The DAC Compatibility List is generated by single posts from the Community on their experiences with a particular DAC.
Having a miniDSP SHD usually means people are using the miniDSP with its integrated Volumio and not as a separate DAC connected to another Volumio device. Which you could and, having one, I fully agree that this works very well. But never added it to the list, because I did not have the feeling this would be of any use.
I think it is worth adding the miniDSP to the list
Please feel free to do so. 
Do make clear though how it is being used to avoid any confusion. Is it just the DAC, or as a system etc.