Twitter is competition to Facebook, Facebook to Twitter, and as we're observing thanks to Twitter coming under new management, Mastodon is competition to Twitter also.
> If you stifle voices on that platform majority of the country won't hear them
This is technically correct (the best kind of correct) but doesn't imply either a legal or moral responsibility on Twitter's part. Freedom of the press has never meant one is obliged to let others use one's press; in fact, it means the opposite.
If people want a channel more unbiased than Twitter [1], they should build it. The Musk purchase has shown how fragile the bird really is when the users lose confidence in it.
You note that them not technically being public is a problem. I think you raise an interesting question: what would a technically public forum look like and who would build it? If the government built it, do we imagine it would run the Post story, or would those in charge of it squelch it because the story is harmful to the active regime? It is, perhaps, best for the government to be out of the business of editorial work; leave that to the press.
(... and if one wants a government-subsidized communication channel that isn't edited by someone else: that's the Internet and we already have it. Then one's goal to combat Twitter becomes building a service as easy-to-use as Twitter that everyone can use; make the press itself as free as possible. I recommend supporting the Mastodon project if that's one's goal. Or put effort into making one of the many, many blogging software packages of the world ever more turnkey. The way to maximize freedom of the press is to maximize presses, not constrain how the freedom can be used).
[1] long experience suggests to me what you actually get when you try this experiment is a channel with different biases, but by all means.