I really dislike the "fire in a crowded theatre" analogy - it was originally used in a court judgement where the accused had spoken out publicly against the draft.
It's honestly madness that this "fire in a crowded theatre" still comes up given where it originated - a court case where exactly the kind of speech that people worry about being restricted was in fact restricted.
To take a both sides view, I don't think anyone actually understands what it means, including the Supremes. Speech can clearly be regulated even in government bodies. You cannot disclose classified information even if you assert freedom of speech. You cannot disclose under a private NDA which the government will enforce that on behalf of a third party --- which is a wild argument if we have freedom of speech as in be free from government persecution. The economic penalty in many cases is equivalent to the criminal penalty (minus the scarlet letter). In fact the government will enforce anti-competes which is the ultimate suppression of speech. So the whole thing is a mess and fubar.
It's honestly madness that this "fire in a crowded theatre" still comes up given where it originated - a court case where exactly the kind of speech that people worry about being restricted was in fact restricted.