is constitutionally-protected free speech.
> mocking presidents
ditto.
The only things that are actively illegal in the US on the list in the article are CSAM and copyright infringement. There are examples that are illegal in other countries, but in the US, vanishingly little speech can be illegal.
The other usage of the term is more of a principle. That citizens of the United States expect its various institutions to uphold the spirit of the First Amendment even though it isn't a legal requirement. That, in an ideal place, your Twitters and Facebooks would adopt a stance similar to the federal government.
There are factors that complect it. Social media platforms are basically entertainment sites and so need to attract advertisers which is at odds with that thinking. Users of the site often want others deplatformed. Standards of legality vary. Then you have cases where the government attempts to influence the moderation standards of a platform, blurring the lines farther.
From my point of view, individual users wanting the opinion of others banned and government influence are the ones that are the most worrying, not least because of what they potentially signify for the future. No right will be maintained if the populace doesn't value its protection.
Incidentally, is there anyone on the compliance teams left at Twitter?