Platonic knowledge is more than mental assent to a proposition; it’s the vision of the whole man, rightly formed by training in geometry, music, and gymnastics (among other arts). The writer of the rather jejune article you cite, claiming as he does that Plato is “wrong” to suppose that “ignorance and not sin is the root of our problems”, needs to reread—or more likely read—The Republic.
What the Platonist calls ignorance is itself in part volitional, the result of a willful refusal to undertake the necessary forms of ascesis, whether the intellective ascesis of dialectic, the physical ascesis of yoga or some other form of “body work”, or the emotional ascesis at work in the assimilation of sacred art. Sin, after all, is a “missing of the mark” (hamartia), a failure of focus, which in turn is attendant upon a vision clouded and fractured by distraction.
The dispute in question is a dispute between perspectives: the first, the Platonic, is jnanic or gnostic; the second, which this writer means to defend, is bhaktic or moral. The distinction is expressed very nicely by Schuon:
“Gnosis objectifies sin—error carried into action—by reducing it to its impersonal causes, but it subjectifies the definition of sin by making the quality of an action depend on personal intention. On the other hand, the moral perspective subjectifies sin by identifying it as it were the agent, but it objectifies the definition of sin by making the quality of an action depend on a rule, hence an external norm.”
Let’s also keep this in mind: Plato is a dialectician, not a dogmatist. He can’t be “wrong”!