Please note that the article headline is misleading. Twitter is not leaving or abandoning the EU. It will continue to be available as normal.

What happened is that the EU has a voluntary Disinformation Code that it has asked large companies to sign onto. A number of them agreed to the Code (Google, Microsoft, Twitch, TikTok...). The Code was essentially a series of guidelines and procedures that signatories agreed to self-enforce internally. Twitter was a signatory and has now decided to drop it.

There is no external authority that enforces the Code. There are no direct consequences for failing to enforce the Code. As the article says, there are a series of other, legally-binding, regulations on disinformation and content moderation that Twitter will need to adhere to.