Yes, that's exactly what it sounds like to me. They have "decided" to press the button, but the body hasn't yet followed the instruction. This situation will of course feel weird and unusual, but I don't think it says anything about consciousness being an illusion, whatever that might even mean.
This just isn't the right interpretation. How can you have made a conscious choice before you were aware you made it. In another similar experiment the subjects were told to choose one of two buttons to press and to take note of when they had decided. In all cases the experimenters were able to not only know when the decision was made, well before the subject noted they had decided, but also could predict what button they would press with some accuracy.
