The advertising model is Twitter's fatal flaw. It puts the fate of the platform in the hands of a tiny corporate mob that are themselves subject to larger mobs.
If "the mission" was truly driving Twitter, they'd drop all advertising and build enough value that some decent percentage of users would pay for it. In a few years, with a lot of work, I believe they could build a $10+ billion/yr business using paid accounts and features. With zero advertising. Twitter is an incredible "channel" for information, marketing, customer support, etc.
But unless they kick their addiction to ads, it doesn't matter if they do or don't believe in free speech, because their advertisers (customers) most definitely don't and they're in ultimate control.