There's the platform versus publisher issue, where companies are arguably trying to be both.
And there's the social media companies already using black box algorithms to control their feeds.
So I'd argue with twitter everything already falls under the latter issue, which doesn't mean it's okay just that this is larger issue for any social media company and it's more complicated than just free speech (which is already more nuanced than I've described).
So yes, once twitter begins censoring opinions (and arguably they already do simply by having an algorithm make a feed for you) then they are equally culpable.
As to the corporation vs government issue, yes I'd prefer to hold governments accountable for these things and I agree they should be the entities that handle this. But I see that as an ideal and meanwhile I care more about practical solutions to help us improve where we are and hopefully get there some day.
I think this is where we disagree. Yes, twitter added their own opinion, but as far as I'm concerned why is that worse than anyone adding their own opinion to it? My point is twitter adding their opinion should be as inconsequential as a random person with 6 followers replying to the tweet saying "this is fake news." The only difference is because people give twitter more credibility than a random stranger, which is the fundamental problem of trust in society. No amount of laws can fix this if you truly advocate for free speech. People just need to educate themselves better about what to believe.