They are allowed to joke about anything. They just aren't allowed to joke about anything anywhere.
If Twitter doesn't want them to make those kinds of jokes on their platform, Twitter is free to tell them they can't make those jokes there. That doesn't stop the Babylon Bee from making those jokes.
And, if Twitter has changed their mind, Twitter is free to tell them that they can now make those jokes on the platform.
This is how the marketplace of ideas works. Twitter is a participant in that marketplace. If you want to say things on their platform, you have to follow their rules.
If Twitter doesn't want them to make those kinds of jokes on their platform, Twitter is free to tell them they can't make those jokes there. That doesn't stop the Babylon Bee from making those jokes.
And, if Twitter has changed their mind, Twitter is free to tell them that they can now make those jokes on the platform.
This is how the marketplace of ideas works. Twitter is a participant in that marketplace. If you want to say things on their platform, you have to follow their rules.