The question is difficult to answer, partly because there is no such job. College-level instructors have different types of terms of employment, and few have the traditional or stereotypical tenured professor-type job.
Being a tenured professor as this is traditionally understood is hugely stress-free. You do as much or as little research as you like, on whatever you want. You don't teach very much, and you teach the stuff you like, mostly if not exclusively to students who have chosen to major in your subject. You have some other administrative duties, but you don't have to take these very seriously. You are well paid, and you cannot be fired. But this describes almost no one's actual job.
According to this Chronicle of Higher Education article, "those with some kind of tenure status represent a mere quarter of all instructors," and the author includes tenure-track-but-not-yet-(and-maybe-never-)tenured people in that category. And that was two and a half years ago. Of the lucky elite among instructors who have tenure, about 2% are fired every year according to the National Education Association. So not that many have tenure, and no one has a guaranteed job for life.
There is another sense in which the job of being a professor does not exist. There is not one such job, not just one sense in which one can be a professor. Some are adjunct, some are full-time but contingent, some are tenure-track but don't have tenure, some have tenure but are subject to post-tenure review, and so on. The level of stress varies enormously. I have it relatively good, but I'm well aware that my position could be terminated if the administration decides that we don't need anyone teaching philosophy, or that I could be dismissed if the administration decides that I am not doing my job properly, and that I can be told what to teach and how to teach it. So even my tenured position has its stress (and I know how much better off I am than adjunct and contingent faculty members).
The crucial facts are that tenure-stream faculty have considerable autonomy and considerable control over when and where they work, even if they are working fifty hours or more per week. The same can not be said for lawyers, most doctors, office workers, business men and women of all stripes, and so on.Two things strike me as odd about this. Presumably Leiter means that tenure-stream faculty members can choose to work at home or in a café or in their office, and can work at night or by day or whenever suits them, apart from the times when they have to be in class or at a meeting. This is a nice feature of the job, it's true. But lawyers, doctors, etc. have a different kind of control over where they work. They can join a practice, or set up their own, pretty much wherever they like. Professors can't do that. It is incredibly difficult to get a tenure-track job, and the number of people who receive so many job offers that they have not just some choice but considerable choice about where they work is surely minuscule. People with tenure only have this kind of choice if they are superstars. I don't know which kind of control over where one works is better, but it isn't obvious to me that professors have the better deal. And the difficulty of moving from one place to another means that it's harder for us to quit, which means we're more vulnerable to extra stress. A law firm that makes its employees miserable might find them getting jobs elsewhere. A university or college that does so can count on many of its employees having little option but to put up with it.
The other thing that strikes me as odd about what Leiter says is that he compares academics with doctors, lawyers, and business-people. Those people typically get paid much more in return for the additional stresses and general unpleasantness of their jobs. That's part of the choice you make when you go to law school or decide on a career in business. (This doesn't apply to generic office workers, but surely no one is claiming that Dilbert's life is enviable.) If you decide to become a lawyer you expect to work long hours on often tedious stuff in return for good pay and a certain amount of social status. If you decide to become an academic, or to try to do so, you expect to have to demonstrate enough ability to pass various academic tests and earn tenure (if you are lucky enough to snag a tenure-track job) in return for a relatively stress-free life doing what you love. You don't expect to get rich, but you do expect, to a large extent, to be left alone to get on with developing and passing on your expertise. If there are no jobs when you get your PhD, or the standards for getting tenure have suddenly been ratcheted up, or the nature of your job (its security, your autonomy, etc.) suddenly changes, then the contract you thought was implicit is violated, and your level of stress increases. And then when someone tells you that you have it easy because academic superstars have it easy you might get a bit annoyed. I personally do have it relatively easy, but I'm well aware that most college and university instructors don't. Let's not exaggerate the stress, but let's not go too far the other way either.