That said, threatening people is illegal all on its own, so who knows…

Not necessarily. If it's just a political threat, that's fine. And like a lot of what Trump says, this is just borderline enough to not explicitly threaten physical violence to the point of criminality. It really needs the witness tampering to become criminal. If he says that he will cause political consequences if Jeff Duncan testifies, that would let the prosecution slap witness tampering on to the rest of the case. Hell, he could be convicted on that alone.