I doubt the amount because guys like TheKoziTwo were involved earlier than Risto. He created the first Monero exchange and was in a much better position to acquire a such a large amount, but IIRC he indicated he had less than half of that. Buying 700k coins during that time on the open market doesn't seem possible without a massive price run (which never really happened. USD high post Mintpal pump was like $3). And I doubt he was making any significant OTC purchases either. The community wasn't that big at the time.

EDIT: back to speculation. If XMR breaks $110 this week we will be on target to $425 by December.