Hmm, so let's assume the writer is a non-religious materialist. Logically, why would it be hard to accept the idea that one's brain is what is generating one's decisions? What the heck else would be doing it? Do you need an experiment, even, to take note of this? Does the time at which the brain generates the results even matter?

Given all this, I would say that "Free Will" is an illusion in the sense that the operation of human brain seems to often generate the sensation that some undifferentiated "self" or "I" is what's doing things and when a person is confronted with strong evidence that their brain must be what's ultimately generating their choices (as well as the sensation of "choice"), it's threatening to this "sense of self", a sense that isn't even logically consistent with our other understandings of the world.