you seem to be referring to the shadow cast by the back rest onto the seat. Your chair.oct seems to contain at least two light sources (suns). This is looks very much like what objpict would produce, which places three sun-like lightsources in the scene, just to have some light.
You can confirm what objects and light sources you compiled into the octree by running
$ getinfo chair.oct
You’ll probably find a rad file named lights.rad or suns.rad in the output?
If you are referring to the aliased edges in general, we recommend you render at 3x resolution in each image dimension, then pass the result through “pfilt -x /3 -y /3 -r .6 orig.hdr > smooth.hdr”. I am sure this is mentioned somewhere in the tutorial, though I don’t have it handy.
I think this is what I was looking for.
I haven’t read through all the tutorials yet, just the Tutorial for Radiance for now.
But this is really helpful, thank you very much.