He wouldn't. Harris is suffering from extreme TDS and it is obvious when you hear him speak of Trump. For all his smug shit about being some enlightened rationalist and "muh Atheism" he sure does have a religious hatred of Trump for attacking the Orthodoxy of his Church.
Then he's being inconsistent in regards to this. If it's fair game for Twitter to remove Trump, then it's also fair game for Twitter to remove him. And if it's fair game for Twitter to ban the New York Post for talking about Hunter Biden's laptop, then it's also fair game for Twitter to silence Sam Harris if it doesn't like whatever he says about Islam. The fact that such a decision might be stupid would not necessarily mean that it would not be within Twitter's rights.