Not so. This is in the same category as committing an armed robbery with a replica gun. He made a threat and that's all it takes to make a "death" threat.
Prosecutors have a lot of discretion when it comes to creative prosecutions. If the defendant has a shitty lawyer, the prosecutor might even win, establishing the legal precedent now lacking.
I disagree, if the virus was proven there, the charge would be terrorism. Terrorism threat is just threatening people into fear, like bomb threat, mass shooting threats, and so on. A similar precedent might be the anthrax fear in the early 2000s.
It's the same a robbing a 7-11 with a replica gun. The "victims" don't know if it's real and were in fear for their lives. He's guilty and was stupid enough to record his action for posterity.
For such a charge to hold up they would have to prove he had the virus at the time of the events. Also I don't think there is a precedent legally for charging someone with spreading a virus as a terrorist.