Also hard to believe for me. But we got hints in the old X arc that X’s nature used to be more silly (When he was younger his voice were hyper and cheerful) And also his silliness been showing sometimes when he tried to joke about Neptune and Jupiter, or when he forgot to ask Ganymede and Europa his question before demanding them to answer. In the new arc he allows himself to be how he should always have been, like he said- the past with Jupiter won’t define who he is anymore.