This is significant, because even though the article quickly dismisses Bitcoin, it is mentioned along with the Chinese yuan. Consider how amazing it is that a newly invented currency, 5 years old, is theoretically comparable with the influence of one which represents a 5,000 year old civilization with over a billion people.
A further point is that reserve currencies are never chosen like some pathetic "basket of currencies" which economists might dream about. Reserve currencies force themselves onto the world stage by sheer economic power from sound fundamentals and global reach. Bitcoin embodies these principles.
Plus, of course, any objection by Krugman is not worth a piece of used toilet paper.
Let's not forget the currency hasn't been there for 5,000 years... maybe 100-500?