There is no threat of violence. Comey claims that he knows nothing about the meaning of those numbers and it's entirely possible albeit highly unlikely. But nothing in his post mentioned Trump so how do they make the connection?
By criminalizing this you are effectively banning the number 8647. Even though this number has appeared millions of times in human history with no relationship to Trump.
This is like the case of the woman who was arrested for saying "you people are next" to a healthcare rep. Well why are they next? Is it because she will kill them or because she is warning them that someone else will regardless of her own actions? Or are they next to win a free coffee machine? She never specified, so you can't say it's a threat. If she had said, "I'll make sure you're next to die", then you would have clear intent to harm. But the state has no authority to read into someone's words meaning where no such meaning has been expressed.