By now, it has become standard terminology in the research community to use the phrase "terahertz radiation" to mean EM waves between 0.1-10 THz (although some people prefer 0.3-30 THz, and there are also other options out there). There is no uniform and universally agreed-upon definition. But the bottom line is this: the term is never used to refer to infrared (e.g., 100 THz) or visible light (e.g., 500 THz).

This part of the spectrum has previously been lumped into the "far infrared", and sometimes called "submillimeter waves", and in the old old days was called "mega-mega cycles". The reason that there is so much confusion about the terminology to describe this spectral range is that it lies outside of the purview of both optics people and RF engineers. It is the "in between" region, which until recently was unfamiliar to most scientists. So there has been a lot of borrowing terms from different communities going on, and these communities have often not coordinated with each other. It's a bit of a mess, but we live with it.

They are referring to the ITU "Terahertz" band which is defined to range from 300 GHz to 3 THz: https://en.wikipedia.org/wiki/Radio_spectrum#ITU

Engineers and physicists have long referred to low THz RF as such and not with optical terms.

Also, there are many low THz applications but nature doesn't make it easy to work with.

https://en.wikipedia.org/wiki/Terahertz_gap

As with the other people that have replied to your comment, the term THz is now used to apply to the region of physics between problems that have been solved by RF solutions and problems that have been solved by optical solutions. The set of technologies that are being developed to deal with frequencies between these two are very different from either. I work with 0.1 - 3 THz which is referred to as THz technology because the research methods and available technologies are unique to this domain, so a unique name is used to refer to it.

Yes I almost thought it was an April fools joke article like dihydrogen oxide dangers.