I got an answer from the article. Essentially this guy was a known critic of the government already having spent time in prison for very direct criticisms. So he posted a dot. A completely benign post and then the following occurred:
“[he] posted a single dot in reply to Supreme Leader Ali Khamenei’s tweet, and that comment was liked far more than Khamenei’s original tweet”
…and…
“Iran watchers say the leadership has felt insecure about the high level of dissent in the country for a while”
So a known critic did the thing you do to your old sibling where you put your finger an inch away from them and say “What? I’m not touching you. I’m not touching you!” and then your sibling is tired of your shit and smacks you very hard.