Unfortunately, I am not surprised because I am no longer surprised by writers' thin skins and peoples' lacks of understanding of the First and Fourteenth Amendments protections of speech. The constitution restricts governments' ability to limit speech. As private citizens, any person or business that does not receive government funding can censor anyone to their hearts delight.

If I saw this as an editor, I would have done as you did but taken it further. I would have checked the writer's profile and if I saw that he or she had published the libel, I would have reported it to Medium as a guideline violation and I may very well have also reported it to the politician.

Your assessment of liability is spot on, and what many do not understand is that only the writer and the owner of the publication would have liability and that Medium is shielded both by Section 230 and by the standard terms of service indemnification provisions. Publication owners do not believe me when I tell them that they have exposure--they say they are just a vehicle like Medium, but Section 230 does not protect them and Medium's terms make them contractually liable. https://medium.com/illumination/defamation-of-character-and-infringement-of-intellectual-property-rights-on-medium-what-every-fa3bd43c08f