You don't joke about breaking the law. They might be listening. They might act on said joke. Which, according to the FBI, they did.
So, if Biden starts saying stupid shit, I can claim the joke defense too? (This is a rhetorical question, you're setting a bad precedent as you're using revisionist history to claim he was joking after the fact when he didn't clarify.) When Trump said this, he was holding a political rally, not a stand-up show. Given the setting and the purpose of the venue, he is assumed to be taken seriously.
This "just joking" defense is actually the lack of a defense.
Try again.
https://www.newstatesman.com/2019/06/when-someone-just-joking
https://sage.thesharps.us/2016/08/10/i-was-only-joking/
https://www.shellypjohnson.com/why-the-phrase-i-was-only-joking-is-usually-an-excuse-for-bad-behavior-that-roseanne-barr-tweet-and-some-other-stuff/