As per the article: "Discord’s terms of service, particularly with respect to the rights they assert over participants’ data, are expansive and very grabby, effectively giving them unlimited rights to do anything they want with anything we put into their service. Coupling that with their active hostility towards interoperability and alternative clients has disqualified them as a community platform."
They even made customizing the Discord client in any way against the ToS. The hostility toward interop is the one thing I really dislike about them. They do so much else well.