Straight up: If you've been charged with a crime, you need to hire a criminal defense lawyer.
(Note, I am not a criminal defense lawyer)
Good luck with your situation. If you believe you may wish to hire a lawyer to help, please feel free to contact me. DISCLAIMER I am a lawyer, but I am not your lawyer unless you come to my office and we sign an engagement agreement. The answer provided here is for general purposes only, and does not presume to correctly address your particular circumstances. My answering this question does not create an attorney/client relationship.
First, you should be careful what you say on here as tt can be used against you. The fact the "threat" was not communicated to the intended recipient is not important it can still be considering criminal threatening. However, based on the quoted language and assuming that is all the prosecution has then it appears to be a weak case for the prosecution.
My response to your question is a generic response and should not be construed as controlling to your case. I can not effectively advise about your case without knowing all the facts. Additionally, my response does not create an attorney-client relationship. You can contact my office to schedule an appointment if you would like to have me represent you.
Criminal threatening in Maine does require that you "intentionally or knowingly place another person in fear of imminent bodily injury." So saying something on twitter that you know that person can't see is likely not criminal threatening. At least you would have a good defense to that charge. The problem is that they could have charged you with terrorizing. In fact, this is what the prosecutor will probably decide to charge you with even though it says "threatening" on the summons form.
Terrorizing requires that a "person in fact communicates to any person a threat to commit or to cause to be committed a crime of violence dangerous to human life, against the person to whom the communication is made or another". There are other elements but the case might come down to whether kicking an ass is a crime of violence (yes it is) dangerous to human life (maybe, maybe not). Of course, some use their terms 'a$$ face,' or 'sh-t for brains' or 'a$$ hat' and if these terms, or terms like them were used in conjunction with the threat to kick that persons ass, it might suggest that threat was really to do violence to the person's head, and thus endanger their life. Naturally, if the alleged victim had their head up their own ass, the prosecution case becomes a lot stronger.
And that is why you definitely need a criminal defense attorney.