Talk of assassination is getting a little edgy, in any context. Not exactly forbidden, but even with “freedom of speech” and all, those who speak of such things are held to the consequences of their words, even if they do not carry out the act themselves. It may be enough for a more suggestible person to believe permission has been given for a contemptible act.
Not all speech is protected in all contexts. Death threats to the President might not be strictly illegal, but they'll get you in a bunch of trouble nonetheless. Like slander and "fighting words", some speech carries consequences that push it out from under the safety umbrella of the 1A.
It's like the saying, "You can ignore reality, but you can't escape the consequences of ignoring reality." So, yeah, "You can say that, but you can't escape the consequences of saying it."
I'm not a lawyer, but I think that goes too far. My view is that you can say any damn thing you want to, no matter how crazy, as long as you don't threaten harm to someone or someone's property. Threatening to kill the President goes too far.