I take his piece to largely be about self-identified bullshit jobs. While many of the jobs he describes, such as transcribing boxes on a form into a system that is never consulted by anyone or hiring someone to guard an entirely empty room, are clearly useless to society, he also describes jobs that have some benefit to someone, but the one performing them feels wasted. While many jobs have various stresses, both mental and physical, he wanted to explore the particular stress of not having much, if anything, to do as well as the stress of doing stuff that is pointless or harmful.

He does categorize the types of bs jobs into several categories:

1. Flunkies. They are largely around to make others feel more important by having others around. Often busy little tasks that don't seem to have much to do with a job description or being useful. Sometimes they sit somewhere just as a warm body.
2. Goons. Largely those without social value. An example might be someone who edits advertisements to make women look unrealistically thin in a way that makes real women feel bad about themselves. These are non-good PR types, corporate lawyers, telemarketers...
3. Duct Tapers. These are people doing something that only exists because the problem is not fixed, for some reason. Example is of a form submission setup that must be routed to a person because they decided to change the format but did not change a program. Or being a "proofreader" of someone whose reports are beyond awful.
4. Box Tickers. They serve to simply ensure that some rule is being observed. Example: someone filling out a preference form that someone has to put in somewhere but the results are never consulted. Others might be highlighting pointless forms, helping people jump through government hoops, asking for funds, or be bureaucrats almost anywhere.
5. Taskmasters. People who assign tasks to others even though those others could easily figure out what they need to do (Type 1) or people who create and oversee bullshit jobs (Type 2): "strategic" and other buzzwordy bullshit.

In addition to those, there are useful jobs that only exist to support useless jobs, such as those cleaning offices of those are useless.

I am not sure where he puts the following, but he had two stories of people where they were essentially hanging out just for an emergency to happen: an engineer managing a rapid response team that never, ever got called to do anything and a caretaker for an elderly woman who was largely fine, but could not be left alone. He writes how they at first tried to find something to do with themselves in a productive fashion, but then simply couldn't take it anymore and quit.

I think he wanted to explore what are the implications of jobs where the one doing it feels useless, whether they were actually useless or not.

The 30-40% of jobs figure comes from both limited surveys where people rate their jobs as well as looking at some statistical growth of jobs. As automation took away a lot of physically productive jobs in the past 70 years, jobs in the information/management/financial sectors grew to fill in that gap, which is roughly 30-40% from the graphs he provided. In his mind, particularly as he is an anarchist, such jobs are majority useless and/or harmful.

For me, I'm simply interested in the idea of finding ways to help people find engaging work because I think society benefits from that the most. I also found it interesting that the ones who had the hardest time being in a useless job are from the working class. While uselessness and boredom is soul crushing for everyone, there are systems of raising that it more palatable.

I would also say that this is largely a starting point for many further explorations of these ideas. This is a narrative of exploration, not a fully flushed out scientific theory. It was definitely written with a popular audience in mind in order to, hopefully, stimulate more analysis rather than a report on findings.