“However, you may still be held responsible for information you provide in commentary or through editing. For example, if you edit the statement, “Fred is not a criminal” to remove the word “not,” a court might find that you have sufficiently contributed to the content to take it as your own. Likewise, if you link to an article, but provide a defamatory comment with the link, you may not qualify for the immunity.”
Twitter is dead to rights.
You really shouldn’t be arguing with someone over a topic that you don’t know squat about and are JUST learning about now.
I would say they did not actually edit his post, just added an addendum. It was stupid, but they could ban or delete his posts totally legally.