You can bet that Trump knows a lot about what if going to happen in terms of justice or else he would never tweet that. His tweet suggests that there is going to be some really high level arrests.
I gave some thought as to how I would handle this situation if I were in Trump's place. Do I just not tweet about it and let the arrests/indictments happen out of the blue? Or do I send out tweet storm after tweet storm, preparing the battlefield, so to speak? So that when it does happen, it's not quite as unexpected and shocking.
There are pros and cons to either approach but I would lean towards the latter. Arresting high level officials in the middle of the night without warning just seems too much like what would happen in a third world country under a tin-pot dictator.
It is better to have events roll out as they are, which is our judicial system carefully building a case and following the rule of law.