This is just a slogan people keep saying, and whilst it's useful fodder for one side of the debate, I'd encourage you to question it. I'm sure you're smart enough to understand the concept of free speech outside of what is protected by the US constitution. That is what people are often talking about, and even though their legal recourse (in the US) is often limited to only preventing government censorship, we are free to have debates about what is right and wrong about allowing or censoring other kinds of speech.
Saying "you're not entitled to X" with reference to current laws is disregarding that people are actually arguing against those laws, or about situations where they don't apply. Even when people are talking about the law, some laws are unjust: quoting present law is not an argument about anything except what the law is.
I'd also encourage you to consider that the writers of the US constitution were in favour of free speech generally, and almost certainly would have tried to constitutionally entrench a broader concept of it than they did, if it were practical to do so. The decision to only prevent government censorship is a practical one - government stopping others from censoring was (is) considered outside of the role of the government, or too impractical to actually enforce. That doesn't mean the writers of the constitution thought that other kinds of censorship were ok. They just didn't think rules about them belonged in a document about the US federal government.
Finally, most countries do not have freedom of speech entrenched in their constitutions, but "free speech" is still a phrase in these places, and they are not talking about the US constitution. Free speech has historically been considered an important right in post-enlightenment societies, regardless of legal protections, and regardless of trendy people on the 'right side of history' saying otherwise. The fact that oppressed people are presently somewhat anti-free-speech is due to the unusual situation of them having friends in high places. This is not usually the case, and throwing out the ideal of free speech to the greatest extent permitted by the US constitution does a great disservice to future oppressed people who are unlikely to have the media and tech companies on their side, as well as present oppressed people who have fallen through the cracks because their flavour of oppression isn't currently in vogue.