You are correct that two wrongs don't make a right, but I think that it is obvious that the threat was not real, only symbolic. Therefore it wasn't "wrong". Meanwhile the original, not prosecuted threat message, was real. It's clear that it shows both vindictiveness and unwillingness to protect certain people.

Sure. If you accept that we give up on equality before the law, one might be prosecutable and the other not.

Some of us prefer not to give up on that though.

Indeed, that’s why selective prosecution is an effective weapon. The consequences are asymmetric and demonstrating selectivity is impossible without exposing oneself to the downside. It’s definitely a stable incumbent regime tactic.