What they said. Apple needed to make sure there were no legal issues in the transition process.According to Marco Arment, about a week or so before the takeover Workflow sent out the 1-page contract that gave them permission to call back to their app. I forget the technical details.
The leading theory was that as part of the acquisition, Apple forced Workflow to get these legal protections in place. it is assumed that companies that did not sign this one-pager were removed from Workflow.