The article is talking mostly about long term interest rates (on 10, 20, 30 yr government bonds). Those are set by the market, by a simple demand/supply mechanism on individual bond issues. Things like QE have some impact, but the impact is hard to gauge. The central banks normally determine only the short term interest rates (up to 3-6 months).

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.