I look forward to someone tying up a trump and showing them the countdown to a bomb under their chair and when it doesn’t go off say, it’s not a real bomb your honor, so not terrorism.
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.