I've not seen it or even read the linked article, but it's surely just rhetorical hyperbole is it not?
Yes. He was joking. Doesn't mean he and his audience do not believe it contains a sentiment of truth, but he was only joking.
It is interesting to have a president who jokes about killing people and still being supported. Interesting to have a President who jokes about everything by which people are rationally offended. And then to say anything is to not be able to take a joke.
The question is, who makes jokes about this sort of thing and why. Well it certainly isn't trying to unite the country to cast a whole political party in a treasonous light, even for laugh.
I would suggest the purpose is to make the audience think that anyone who is a Democrat is un-American. He wants people to view his detractors poorly to shut out their ideas and criticisms. And the rhetoric works. The jokes work. If you say something, well, then you just can't take a joke, can you?