Is a nice looking frame.. really like the section round the seat tube.
I can't see why you could'nt run mechanical shifting on that, the only thing which jumps to mind as a possibility is internally it has routing which features bends which are to extreme for cables.
All this would mean is that you would have to run the cables externally on the exterior of the frame. Fact is some people have to run Di2 on the frame, so why not consider doing the same for cables.
Would be interested to know why the mechanic thinks mechanical shifting can't be done on that frame.... Just over a year ago, people were saying you could not run Di2 on MTB frames, now K-Edge sell a conversion.