You're making that up. This is everything he said about the slogan: "3. Silence about racism is violence. But elevate the voices of the oppressed over your own." There is absolutely nothing to suggest that he is forcing a literal meaning of the phrase.
Originally you said "I am highly skeptical that a significant number of people profess to believe this." Now that your claim has been proven false, you're moving the goal posts to "people don't believe in its literal meaning" and trying to pretend that McWhorter suggested otherwise. Yet you accuse McWhorter of strawmanning. Ironic.
Actually there is: the inclusion of the words "about racism". "Silence is violence" is a catchy slogan. "Silence about racism is violence" is less catchy, so it is not at all implausible that he did not intend it to be read as a slogan, but rather an actual literal assertion.
One should generally try to figure out what the author's intended meaning was, rather than twist the meaning into something that's easy to argue against but the author would never approve of.
Likewise, "Silence is violence" doesn't literally mean what it says. What the people who recite it actually mean, I suspect, is that if you do not actively speak out against racism then you are complicit in it, which is a not-entirely-unreasonable position.
But McWhorter was not talking about the slogan. His wording presents it as a literal truth.