…although the critique applies to lots of moral philosophy as well (which is also a moral etiquette manual for the bourgeosie).  As the abstract at the start puts it:

The New York Times advice column, where snitching liberal busybodies come to seek absolution, is more than a mere annoyance. In limiting our ethical considerations to tricky personal situations and dilemmas, it directs our thinking away from the larger structural injustices of our time.

And from the body of the text:

In one typical case, an Ethicist reader writes in to ask, “Are People Who Read Magazines Without Paying ‘Stealing’ Their Content?” At their local Barnes and Noble, it seems, this person has witnessed other customers who “grab multiple magazines and take them to their table with their coffee” to read, only to later “put the magazines back in the racks without paying for them.” The questioner’s indignation is palpable, and they stridently complain that their fellow bookstore-goers are “‘stealing’ content that is meant to be purchased.” 

This person may be the perfect example of an Ethicist reader. They’re peeved about a perceived transgression that doesn’t affect them in any way or really harm anyone (the store gets its magazines back and loses nothing), and they want the entire world to know about it. Underlying the mentality of the question is a core belief in the sanctity of property. The idea is that even immaterial things like reading an article must always be paid for in a market transaction and that no circumstances can justify sacrilege against that holy principle. The correct response to this kind of question would be, “What on Earth are you talking about?” followed by “Don’t you have something better to do?” 

But because The Ethicist—and The New York Times as a bourgeois institution—fundamentally shares the belief that property is sacred, this is not the answer the complainer receives. Instead, Appiah tells them that reading the magazines is acceptable because Barnes and Noble itself hasn’t opted to “crack down on the behavior” and is “acting as if what they’re doing is OK.” In other words, the appropriate conduct for the situation is whatever the business owner decides. Property law has resolved the ethical question

And similarly:

[Readers] often write to Appiah about the problems they’re having with members of the lower classes. For instance, one reader asks, “A Homeless Man Sleeps in the Lobby of My Apartment Building. What Should I Do?” The reader, who lives in a “lovely old apartment in a relatively upper-class suburb of a major U.S. city,” is troubled that “a homeless man has been sleeping in the unlocked vestibule of my building,” even though he “hasn’t directly harmed any person in the building” or done anything especially offensive beyond perhaps once eating someone’s delivered food order. The Ethicist tells the reader that “your safety matters,” implying that the homeless are inherently dangerous, and suggests talking to others—although not, notably, talking to the homeless man himself or directly offering him help of any kind. The Ethicist also suggests “putting a lock on the vestibule.” (There’s the fixation on protecting property again.)

Citing Trotsky, the authors write:

In Their Morals and Ours, he draws a clear distinction between morality in general and “bourgeois” morality. As he describes it, the latter not only has its origins in class inequality, but serves as a mechanism to protect that inequality: 

"Morality is one of the ideological functions in [class] struggle. The ruling class forces its ends upon society and habituates it to considering all those means which contradict its ends as immoral. That is the chief function of official morality. It pursues the idea of the “greatest possible happiness” not for the majority but for a small and ever diminishing minority. Such a regime could not have endured for even a week through force alone. It needs the cement of morality. The production of this cement constitutes the profession of the petty bourgeois theoreticians and moralists. They radiate all the colors of the rainbow but in the final analysis remain apostles of slavery and submission."

Once again, we can see exactly this dynamic at work in the pages of The Ethicist. There is a “small and ever diminishing minority” for whom the advice is intended, and whose worldview is reflected in the columns. The “majority” appear as a never-ending source of trouble for that minority, not as subjects in their own right. At every turn, the bedrock assumption is that the actions of poor and working-class people are likely to cause a problem, one that Appiah and his sage advice will help to overcome. The idea that the reader might be working-class themself never arises. In this moral universe, working people’s behavior needs to be monitored and policed. Gambling nannies need to be fired (if only because they might one day be tempted to steal!), and homeless men need to get out of the lobby so polite New York Times readers don’t have to feel uncomfortable. The fundamental message is that people should know their place, and the behavior appropriate to it. With such “cement” is the elite liberal order, the class order, maintained.

The article is full of other apt examples.  Appiah's advice, to be sure, is mostly good bourgeois etiquette advice given his class position and that of most of his readers.  The authors of this critique, however, reveal their own moral triviality after earlier offering another  quote from Trotsky's Their Morals and Ours, where he "blasts the self-righteous armchair moralists of his own time":

The chief traits of the prophets of this type are alienation from great historical movements, a hardened conservative mentality, smug narrowness, and a most primitive political cowardice. More than anything, moralists wish that history should leave them in peace with their little books, little magazines, subscribers, common sense, and moral copybooks. But history does not leave them in peace. It cuffs them now from the left, now from the right.

"What a perfect description of The Ethicist and its readers!," the authors proclaim, and then proceed to identify the "'great historical movements' to be involved in, from the struggle to defund the police and abolish mass incarceration to the cause of the Palestinian people."  But the sruggle to "defund the police and abolish mass incarceration" is itself idiocy of the infantile left, since the police and incarceration exist as they do precisely because of the class structure of society:  eliminating the former while leaving the latter untouched is just an invitation to mayhem and misery.  The "great historical movement" of the present is against authoritarian capitalism, from China to Russia and now the United States, full stop.  Everything else is a sideshow.  Too bad the authors marred their otherwise apt critique of Appiah with this silliness.