So, if someone said they wanted Brad Pitt to save them from the police, how likely do you think someone will take that seriously?
I would put it in the 0.001-0.00001% possibility. If you fear the mentally ill and make policy choices on that fear, you would necessarily have to ban free speech to mitigate such low risk.
In this case I genuinely believe the post was borderline criminal and grossly irresponsible given the situation the world was in on March 20, 2020.