In typical secrecy, Apple didn't have much to say about joining WPC. Apple told Business Insider in a statement:
Apple is an active member of many standards development organizations, as both a leader and contributor. Apple is joining the Wireless Power Consortium to be able to participate and contribute ideas to the open, collaborative development of future wireless charging standards. We look forward to working together with the WPC and its members.
The Apple Watch uses a modified version of Qi for wireless charging, a WPC exec said in 2015. While joining the group doesn't guarantee wireless charging for the next iPhone, it looks to be a step in the right direction.