First of all, the fidelity of Koolas work cant be achieved with any dynamic solution UE4 is providing right now. DFAO is too low rez and standard AO from post has no directionality, but which is what you want. So you have to aim for achieving this in Lightmass.
Second, Koola said that he is not using the Lighmass AO feature that can be found in the world settings (which might help with that).
Stationary skylight “could” work since it provides these kinds of shadows and saves them into bent normals which could be very nice, but you have to test a bit and see how its performing.
So my guess is you a) definitely need this reflector setup to get these nice smooth contact shadows and b) you might need to put indirect lighting quality to 2 or 3 and reduce the smoothness to something between 0.6 and 0.75.
If you dont reduce the smoothing, the nice contact shadows basically get smoothed away and you wont be able to see them anymore.
Let me know if this helps 
Cheers!