If the comment were demanding that substack remove the article, or if the commenter was trying to get the author fired from his job for this article, I would consider it against the idea of free speech. Note that this does not equate to a demand for any legislative change or government action.
The view expressed in XKCD 1357 is that there would be no free speech issue since it isn't the government trying to prevent the author from making his article available to those who wish to read it (again, no one is asking for a right to an audience or right to be protected from disagreement) or threatening his livelihood as retaliation for publishing the article.
