It's a good idea to leave X, considering the values of the Free Software Foundation and how they don't exactly align with X's profit-driven model, and the spread of internet garbage.
Actually the announcement is by the Free Software Foundation Europe, an organization that empowers users to control technology.
I think either leaving a hostile platform or staying to promote ideas which run against it are reasonable choices. But what the article doesn't say is why they (as an organisation whose mission is to protect and promote free software) should choose to leave now, when from what I know, Twitter has always been a non-free, profit-driven network.