The idea that you can say whatever you want, and that no one can stop you; is not the idea behind free speech.
Actually, no, that's pretty much it.
The "no one can stop you part" is particularly relevant to government only because they're the only ones who can shut you up with violence and it sometimes be acceptable.
And before you go all libertarian about it being Twitter's platform, remember whatever argument you make applies to network neutrality as a whole.
Free speech is the same as freedom of conscience. The ideas in your head are yours, no one can force you to have different ideas, and you can share your ideas with whomever you please. That is what free speech means.