IANAL, but as I understand it, in order for the courts to consider this a "true threat", the government must prove (beyond reasonable doubt) that the it was intended to be taken seriously (the subjective test), or a reasonable person familiar with the context would interpret it as a serious threat (the objective test).

Here's a case of almost the same thing happening in CA, from December of 2012: http://www.popehat.com/2012/12/18/the-true-threat-doctrine-i...