You're right that it's better when no single person or small cadre can unilaterally throttle access to the public square.
However, I don't think it was that unclear at the time that deplatforming Trump violated Twitter's nominal commitment to free speech. What made it a difficult decision was Twitter's workforce, which was composed almost exclusively of progressive Democrats. The tribal drumbeat to drive out their political adversary must have been deafening.
Had Twitter had a commitment to hire people with actual diversity of thought, maybe there would've been more principled defenders of free speech to push back.
https://nypost.com/2022/12/16/twitter-employees-donated-almost-exclusively-to-democrats-in-2022/