I have only ever seen fiber optics described in terms like "total internal reflectance" so it sort of blew my mind to consider them as waveguides for terahertz em fields, sort of obvious in hindsight, but most popular literature really appears to avoid thinking about light as a field preferring a photonic particle approach.

Which was one of many topics of that channel that also blew my mind, that photons might not exist, not as a particle anyhow, a photon(quantized amount of em field energy) is just how the em field interacts with matter. However because that is how we detect light, via it's interaction with matter, that interaction is probably the most important aspect. But it is totally possible to have sub-photonic levels of em-field.