The description given in Chapter 6 of Rendering With Radiance for the behaviour of mkillum is incorrect (see section 6.8.3, p379). Mkillum sends out rays to sample the scene even with the ambient bounces set to zero during the creation of the illum material - see the d and s parameters in the mkillum man page.
Thus, a rendering generated using the illum surface created in the following commands:
mkillum -ab 0 scene.oct < window.rad > mki0win.rad oconv office.rad mki0win.rad cie_ovc.rad out.rad > mki0scene.oct rpict -ab 0 [options] mki0scene.oct > mki0ab0.pic
will be equivalent to a rendering generated using ambient bounces set to 1 with the ordinary window description:
rpict -ab 1 [options] scene.oct > ov_ab1.pic
This can be seen in the false colour renderings shown below where, using the above commands, we have computed the surface luminance for the direct sky component of illumination under standard overcast sky conditions (luminance of the sill wall and ceiling is zero since they cannot 'see' the sky):