Now this is actually an interesting argument. Are threats of violence, even if they are not credible, still a violent offence?

Legally - yes. Threats of specific violent crimes against specific persons are considered a violent offence, even if the offender has no means to carry out the threats.

However, does that argument really hold water? If I were to threaten to fly Joe Blow out to Saturn and shove him out the airlock, is that a violent crime? It's a threat of a specific crime against a specific person. But it's not credible. According to the letter of the law, I would have committed a violent crime. But realistically nobody would care. Just like how a 10 year old threatening "I'm going to kill you" to a bully on the playground isn't a credible threat.

So, I'd say it could be argued either way. The letter of the law is that such a threat, even if it's not credible, is still a crime. But from a practical standpoint, it's frivolous to prosecute non-credible threats the same way you would prosecute credible threats.