I teach mathematics at a large R1 state university. His comments echo my experience. Online is really different, both for the instructor AND for the student. Most instructors don't know how to teach online, and most students (even very good ones) don't know how to learn online.

- Reading the room: students, by default, turn off all their cameras and microphones, despite my request (we cannot require this) that they turn them on if there are no privacy, bandwidth, or other issues. Whereas in a classroom, a student can signal confusion merely by frowning or half-raising a hand, I now need to ask a prepared question or give them a real pause.

- Avoiding distraction: there is not much one can do. The things one can use to hold attention in a live in-person lecture -- telling bad math jokes, etc -- don't translate easily. In particular, if you can't see or hear your audience, it can be a little dangerous to try to be funny.

- Establishing command: did not have this problem. I would in any case say this differently: the issue is building rapport with students. Since we switched to online in March, I had half a semester to get to know my students (small class, 20 or so) which really helped.

None of this says online is a non-starter, but it should be organized very differently. One can do recorded lectures and live structured problem sessions (i.e., flip the classroom), or give students more preparatory activity to do, etc. Many of these are sound things to do in a live in-person classroom anyway; online is just not so forgiving and exposes many of the flaws of the traditional lecture format. And students need to be taught how to learn in this environment from the get-go.

Finally, I'll say that the technology has a ways to go. It would be really great if multiple people could talk over each other as if they were in the same room. If video degraded more gracefully (because many students do not have stable enough connections at home) while maintaining audio. If various apps with white boards gave students the ability to "see" your previous boards. (A couple programs do this, e.g., MS Teams, but AFAIK not Zoom.)

And he didn't mention the nightmare that is assessment. I taught a small class to a group of bright, emotionally mature juniors and seniors. Teaching first year students is a completely different story. This is something we're working hard on, with a summer to prepare instead of just a week (our transition happened over spring break). But it's not easy.

Edit: I suppose one way to summarize the above is to say that all the issues can be addressed, but online makes the energy barrier higher (or friction greater, whatever analogy you like) for both instructors and students.