What does it even mean for consciousness to be an illusion? An illusion implies that someone is being fooled by consciousness, but who is the someone being fooled if there is no underlying true self?
The illusion is that these abstractions are evidence of something qualitatively more advanced than simple neurons interacting in a way that's beneficial for the organism. Which isn't unusual, we've seen the same kind of reaction when people proclaim that they couldn't have "come from monkeys," or any other scientific discovery that knocks humans off the pedestal they've placed themselves upon.
A society of only p-zombies would call p-zombieness consciousness and they would be no less correct than us.
There were some experiments in which subjects were asked to push a button whenever they chose. The results showed that the impulse to push the button arose before the subject was aware of having "decided" to push the button (I can't recall how they managed to detect and measure this).
At any rate, I think that's what the parent is referring to.
Neuroscience actually confirms this :-) Brain scans show that conscious thoughts are reflections of subconscious processes doing all the work, AND that the rational mind is very good at creating post-hoc rationalizations for explaining "reasons" of why you arrived to the decisions you took. Most of what we call "reasoning" is about creating narratives to save our self-esteem.