Freedom of speech has nothing to do with private platforms that are subject to market effects. Zuckerberg's claims that corporations can and will guarantee free expression is obviously not true.
Freedom of speech as a legal concept certainly impacts platforms. Platforms can choose to enforce their own rules that are more restrictive than strictly what the law requires, and in doing so they are making market-based choices. However, they can’t choose a set of rules less restrictive than what is legally allowed in a jurisdiction and expect to still be able to do business for long. You could claim that’s a market decision too, but it’s one driven by laws around freedom of speech.