There are two material differences in your scenario and what happened:
Physical restraint and everyone knows a bomb goes bang. That’s an overt threat of death whether the bomb is real or not.
Sending cornstarch in an envelope with the words: You’re a bad person and now you’ll get what you deserve...could be construed as a statement that all that person deserves is some corn starch.
Because they are a bad person.
If this person is found, arrested and charged...the DA will either cut a deal for malicious mischief or otherwise not bring it to trial.
There was no overt threat made.
When a person opens the envelope with white powder, they do not know at the time is corn starch, that is a moment of terror and were i on jury I would interpret the act of preparing and mailing the envelope as terrorism.