Facebook agreed to not share WhatsApp data with other products as part of the agreement that gave them the EU regulator's permission to take over WhatsApp. If they shared data, it would have been a breach of that agreement.
Could, for example, Facebook be forced to sell WhatsApp (in the EU)?
List of highest fines: https://dataprivacymanager.net/5-biggest-gdpr-fines-so-far-2...
AFAIK, Giving OK to Facebook buying WhatsApp was mostly an Antitrust issue.
http://fingfx.thomsonreuters.com/gfx/editorcharts/EU-GOOGLE-...
They’re ignoring GDPR and a variety of other laws; why would this agreement be different?