Terahertz can still penetrate the body, but because of the low energy, it's non-ionising, and harmless, unlike, for example, X-rays.
Wow, there is something that is actually true! Terahertz radiation, the part of the electromagnetic spectrum between/at the overlap of radio and infrared, indeed has potential as a non-ionising alternative to X-rays for imaging through various kinds of solid material including bodily tissues (although only on thin scales), and indeed at least in dentistry also seems to give better images. The first experiments into these applications were already done in the 1960s, but they are very difficult to produce in a human setting, with such technologies only starting to emerge in the 21st Century.
The magical healing light woo, of course, it total nonsense. Indeed, non-empirically-backed mathematical models posit a very minuscle risk that, under very unlikely circumstances, it could cause bubbles within DNA helices that would interfere with nuclear function by way of non-linear resonances.