And freedom of speech also includes to the rights of private individuals and companies to determine what they share on their own platforms, and what they can or cannot be compelled to host or say.
Twitter is not a product designed to reflect the beliefs and opinions of the company. If it was some sort of editorial platform that argument would hold a lot more weight.
It's impossible to deny freedom of speech (or freedom of association) to a company without denying those same freedoms to the people forming the company, or its customers or users of its service. That's one of the reasons companies do have freedom of speech, and exist as quasilegal "entities" in and of themselves. Rights don't cease to exist in aggregate.
Any legal pretense one could use to deny freedom of speech and expression to a company could also be used to deny any group - political parties, religions, specific races or genders, the press, etc.
