I think that this references the same incident described by Shelley Fisher Fishkin in her essay Mark Twain, Race, and Huckleberry Finn. Fishkin notes that in Huckleberry Finn, Twain "explored the subject of racism through satire and irony" a strategy he developed "when a direct expose of racism that he wrote was censored" - very similar to the wording of the article cited in the question. Fishkin describes the events as follows.
As a young reporter in San Francisco in the mid-1860s, Twain
witnessed an incident he considered outrageous: several
policemen stood idly by, apparently amused, as young white hooligans
attacked a Chinese man who was going about his business. Twain’s
publishers refused to run the account he wrote of the incident, caring
more about not offending the paper’s subscribers (who shared
the police’s prejudices) than about the truth. Twain quickly
learned that exposés of racism in San Francisco would not be printed
in newspapers there.
The incident concerns racism rather than slavery, but it seems to be relevant to the ironic form that Finn took, and definitely involves censorship.
In The Autobiography of Mark Twain, the text is quoted as:
One Sunday afternoon I saw some hoodlums chasing and stoning a
Chinaman who was heavily laden with the weekly wash of his Christian
customers, and I noticed that a policeman was observing this
performance with an amused interest—nothing more.
According to Mark Twain’s Racial Ideologies and His Portrayal of the Chinese, an article published by Hsin-yun Ou (Concentric: Literary and Cultural Studies 36.2Sept. 2010: 33-59)
Twain rewrote the story with a satirical strategy of irony, and
published it in a paper in the next state and in a national
magazine. In “What Have the Police Been Doing?” (Territorial
Enterprise [Virginia City], Jan. 16-18, 1866), the narrator,
posing as the policemen’s loyal friend, satirizes corrupt
police officers who constantly victimized the local Chinese