You may have a point, except you haven't mentioned how many city's worth of power non-btc transactions currently require, not to mention the additional inneficiency of hundreds of thousands of staff which could possibly be reduced in number because of the distributed ledger.

I really don't think things are as clear cut as the article implies, but I dont have numbers for the amount of energy consumed by modern electronic value transaction and storage.

Edit: also, "city's worth" as a measure of electricity, as in the article, is a terrible choice because it immediately biases the reader and does not convey much useful information. My GPU maxes at 300W; the article provides no way of conversion. Never mind that a small city may be a suburb using one hundredth of the power used by a small city center full of servers and a handful of skyscrapers.