From the article: "They stumbled upon something in the app that mentioned there were paid plans, they went to the website and saw there was a subscription you could purchase, and then turned around and demanded we add IAP."

This is the part that Apple took issue with. Either they remove references to the external paid service, or they provide a way to pay for it from the app, with the usual fee. It's misleading to characterize this as "Apple forces a free app to become paid". Apple is just doing their usual thing of requiring that payments for features of an app cannot bypass Apple's payment system.