I watched the event live, and nothing about it was humorous, least of all the suggestion that the United States would even consider for a moment undoing one of our strongest Constitutional protections, term limits on the presidency.
I'm not saying I agree with the sentiment, or think during the current discourse it isn't a very touchy subject to try and pull humor out of...but if there's any event to take the President's words at the highest chance of being a 'joke', this is that event.
You may not think he’s funny, or you can think a joke is in poor taste, but the correspondents dinner is one where jokes are told. Nothing there is supposed to be taken too seriously.