It's interesting that they used lectures as the test. I'd guess that most people in day long videoconferences experiencing fatigue are not attending lectures, but rather attending meetings for work.
Having taken a bunch of classes the past couple years in person and via videoconferencing, I can attest that the subject certainly impacts if the online meeting/lecture is fatiguing. I had some classes which I enjoyed remote, while others I wished were actually offered in person.
As it relates to business/work, I'd venture to say the fatique is just the pure number of meetings. Having supported videoconferencing for the past 20+ years, the fatigue is likely do to back to back meetings, in ability to get a break, senseless meetings, etc. Videoconference adoption has really made it super easy for people to mismanage meetings and be inconsiderate (unintentional, or otherwise) to those attending.
PS: I'm glad Webex was mentioned in the article header. It is far to often overlooked and a much more suitable and quality product over Zoom and MS Teams. Before the haters chime in, those who dislike it please make sure you have used the latest version, not the one from 20 years ago. Please also do back-to-back comparisons between the 3 platforms as well. It will become clear which platform offers a higher quality experience.