First, the article was saying GreenpeaceUSA paid $5M to him, not more than $5M or Billion! don't make a random assumption. There's a research that calculate it needs at least $5B to $20B to attack Bitcoin, $5M is nothing to compare with $5B.

And even they have such huge amount money, high performance mining rigs aren't available, it takes time to produce it. This make them need to spend more and more than that, moreover mining difficulty will increase and it make attack to Bitcoin is never be achieved.