Corporations are not people and are not entitled to free speech protections granted to American citizens.
Corporations are groups of people offering others a good or service. That would fall under the third clause, free association.
Twitter, like this site or any other, has every right to promote or demote any content they wish for their business, it is none of the government's business.
They built the platform for speech, they can choose what to let on that platform. Private property.