I would tend to agree that addressing potential death in a reddit post as a professor is a bit questionable. Whether you like it or not you represent the university, and your online presence is expected to fall in line when you present that association. You don’t have to pretend like nobody is going to die, but did you consider it may be better to leave the delivery of that message to officials instead of an anonymous social media account? We tend to give our professors a bit of undue credit and assume they know something we don’t, even when they say they don’t (healthy doubt, though generally unnecessary), and to say “yeah I’m a professor, some of you will die” abuses that trust relationship to project your belief on what will happen.
I don’t disagree with anything you said, but criticism of the delivery and wording is definitely fair