Did you even read the article? It's clear that while not the main theme, racism is a subtext of many of his stories, but the references may not be clear to people who are not old enough to remember blatant racist laws and practices in the U. S.:
"Early on in Lolita, Humbert Humbert learns that his first wife, Valeria, relocated from Europe to the United States, where she became a subject of an ethnological experiment. The project involved research on “human and racial reactions to a diet of bananas and dates”; this vignette is emblematic of the United States’s obsession with understanding race through “science,” and is tinged with overtones of Nazi eugenicist experimentation. A forgotten target of Nabokov’s satirical eye, the United States’s obsession with racial purity and the policing sex almost exclusively toward that end, is arguably a subplot of Lolita."
"Indeed, one could argue that part of why Humbert Humbert so successfully evades discovery as he takes Lolita from motel to motel is that white men, even white pedophiles, were not surveilled with the same force or regularity as black men engaged in the “sex crime” of miscegenation. The Mann Act, which made transporting girls and women across state lines for the purpose of “debauchery” illegal, looms in the background of Lolita. Humbert Humbert even refers to the Mann Act, though, in true Nabokovian fashion, his quarrel with the law is aesthetic in nature — “I deplore the Mann Act as lending itself to a dreadful pun.” The Mann Act was often employed to target black men who were carrying out consensual relationships with adult white women."
"...the only time Humbert’s intentions with Lolita are potentially foiled is when he is mistaken for being Jewish, based on a misspelling of his last name by an employee at The Enchanted Hunters hotel. At first, Humbert is told that there are no rooms available, but when he explains that his name “is not Humberg […] but Humbert,” a room suddenly opens up..."
Racism was institutional in much of the rest of world at the time, and overt criticism of it in any form was not likely to be well-received anywhere. Had Nabokov been any less elliptical about his racism references he would have had much more trouble getting his writings published and distributed. Compare the backlash against Mark Twain's in America following the publication of Huckleberry Finn, which raises racial hackles to this day.
Dr. HotSalt