Note that even as QE is being slowly reversed in the United States and the short term interest rates have been raised - the 10yr, 20yr, 30yr treasury yields are still tanking like crazy. So it is not the fault of the central banks, which is the point of the article.
In short - people are buying bonds which eventually drives the yield below zero. There's no rule that says "a bond can only be sold at below the levels that the 0% yield implies", therefore brace yourself for the possibility of breaching this level. For any currency, including USD.
But if the short-term rates are manually set by central banks to be artificially low, wouldn't that be the primary driver behind negative long-term interest rates even if the exact number is determined by supply and demand? The article is talking about natural drivers like "negative time preference" which just sounds wrong.