I would say Gina Rinehart is more powerful. She is worth currently 12.7 billion US dollars.
Ahrendts didn't inherit her daddy's wealth, she built it on her own. Besides, Rinehart is the last of the 19th century-style robber barons, the future will not be created by mineral prospectors and miners. There are women richer than Ahrendts, even richer than Rinehart, but Ahrendts will have more impact on the future of humanity.
This retitling of her job is not a demotion. Apple recently rebranded Apple Stores to just "Apple." This new job title is a recognition of her consolidation of all sales operations, under her command. Apple has no other executive leaders who are responsible for sales.
Ahrendts is recreating Apple stores as something beyond retail. Look at the first new store opened under Ahrendts' redesign:
Mall of the Emirates, Dubai. It has a 6k rez video screen and
a forest. A while ago, Ahrendts gave an interview where she described her goal to take Apple stores beyond retail, into a lifestyle experience destination, more of a community center for Apple users, than retail outlet. Look at what she did as CEO at Burberry: she took an aging British fashion brand that was notorious as a lower-class symbol, and turned it into a lifestyle, including a music channel. That's why Apple hired her.