I think what the comment above is saying is that it's not about whether or not speech on Twitter is protected. It's that the government isn't supposed to act to restrict speech in any manner that doesn't cross the lines he listed.
If I'm remembering correctly, there was a big court case because Trump was hiding critical responses to his tweets on Twitter. The judge ruled that Trump violated the 1st amendment even though Twitter is a private company ("private property"?).
This is because the 1st amendment not only protects speech, it restricts government attempts to control speech (or at least that's the argument that would be made).