The problem with this article is that it focusses on a single aspect of brain physiology without considering the determinism (or the acausality) of other events. The main problem with free will existing seems to be that our actions are either caused or uncaused. If they're caused, the classic determinism issue comes up. If they're uncaused, we can't influence them. The existence or otherwise, or the nature, of the readiness potential has no bearing on that.