Apparently it is the size of the light dome. When the dome is small, the shadows can indeed be soft. When the dome is large, then there is no softness in the shadows. IBL dome is infinte size, so there can never be soft shadows from the IBL lights.
I'm actually getting the same effect from normal omni light too. If the light is close then the shadows can be soft; distant omni = no soft shadows.
When the omni is at 50,50,50 then the shadows look nice and soft (softness 100). Put the same light w/ same settings at 5000,5000,5000 and the shadows turn hard.

Friends don't let friends use booleans.