Matt Taibbi supposes (in that paragraph) that their motivation in censoring scientific discussion of Ivermectin's effectiveness was to be "opposite to Donald Trump's comments" and thus to conform to one's own ideological tribe ... because if you don't conform, you risk being cancelled, don't you? And he bemoans that politics has corrupted real science and driven actual scientific debate underground.
Refer also to the Solomon Asch conformity experiment which occurred during a previous episode of cancel culture called "McCarthyism".
There are other theories. Some people have supposed that the motivations have to do with the money that big pharma would lose if they couldn't sell their vaccines because a safe and effective alternative was already available and proven, and therefore the conditions of the emergency use authorizations for the vaccines would become void. But nobody has any smoking gun evidence here that I'm aware of, it's just a plausible motivation.
Some people might believe so strongly that vaccines are the only possible savior of humanity that anything which detracts from the success of the vaccine campaign will be devastating, and so they do what they can to shut it down. Bret argues this is illogical if Ivermectin works because all forms of immunity work together to build herd immunity. But logic is unfortunately lost on far too many people.
More far fetched ideas include influence campaigns from foreign powers who aim to see America defeated. If they can influence the right people in the right way at the right time, they might be able to prolong the pandemic in America.
I'm sure there are even more hypotheses as to why such censorship is being attempted.